Can you tell us how you measured it? The results don't sound right.
Actually these transistors can be tested in diode mode. A good one of these transistors will measure about 0.65v between B-E and also between B-C in diode mode.

BTW, in your picture of the board, U11 looks cooked by the marks around pins 2-5, possibly U4 too with the bubble above pin 4. Do any of the ICs on the board get hot after a few minutes?

Do you have one of these meters: ?
https://pinside.com/pinball/forum/topic/kiss-pinball-79/page/3#post-3624061
With this basic 2N3904 NPN transistor, measure it in diode mode putting the red meter lead on the 'B' pin. The black meter lead to 'E' will probably measure high 800 - low 900 and similar result to the 'C' pin using a cheap multi-meter like the pic above.

A fluke brand meter will probably measure in the range of 0.64 - 0.70 in diode mode between B-E and also B-C.

If the transistor is out of circuit, reversing the meter leads with this NPN transistor should give you open circuit readings.

You can pull this Q3 transistor off the board for now - it isn't critical for generating tones. A working board will produce some wrong tones without Q3, but you're only getting buzzing and the board has other issues.

Having said all this, if the only test tool you have is a multi-meter, it isn't really going to cut it in diagnosing this board. You might want to consider contacting barakandl and seeing if he can repair/replace this sound board. He fixes a lot of these.

Q5, Q6, Q7 and Q18 are not connected to any coils.
Q8 drives the Treasure Chamber Saucer
Q9 drives the Left Pop Bumper
Q15 activates the Flipper Enable relay on the SDB (you should hear it click)
Q19 activates the Coin Lockout coil on the front door

I'll bombard you with a lot to test here to isolate the issue more quickly. Try and do the following step by step.

Put the game in solenoid test mode. Measure the voltage at pin 19 of IC U2 on the SDB. It should be around zero volts - this signal being low opens communication from the MPU board to the SDB. If it measures steady logic high (3.5 - 5V) then check it at the source (pin 19 of U11 at the MPU board). If it's measuring high here at the MPU board then this is a problem. If it's low, then there's a connector issue from MPU J4 pin 10 to the SDB J4 pin 7.

If the above signal pin 19 of U2 on the SDB measures ok (Low), try to manually activate solenoids with the logic lines. During the solenoid test *very briefly* ground the following pins at U2 on the SDB:
U2 pin 8 should fire the Treasure Chamber Saucer
U2 pin 11 should fire the Right Pop Bumper
U2 pin 15 should fire the Left Slingshot
U2 pin 16 should fire the Right Slingshot

If these solenoids don't fire, the issue is on the SDB probably with the ICs. If they do fire, then check the solenoid select signals from the MPU board to the SDB (stay in Solenoid Test mode).
With your logic probe, probe pins 20,21,22,23 of U2 on the SDB. When solenoids are not firing these pins should indicate High. When solenoids are being activated, these signals at different times will pulse Low (depending on the solenoid being selected to activate). If one or more of these signals never pulse low, check these signals back at the source (MPU board at U11 on pins 13,12,11,10 respectively). If they all pulse at some point on the MPU board you have connector issues from the MPU board J4 pins 1,2,3,4 to the SDB at J4 pins 3,4,5,6 respectively.

Good luck!

*sigh* why couldn't Alltek keep the primary chip location numbers standard.
If you have the "L" revision MPU board, replace everything I said above about U11 on the MPU board with U2 on the MPU board. And yes, pin 1 of U2 (6821 chip) is at top left corner and counts anticlockwise.

Unlikely, but you can measure them if you want. Set your meter to resistance ohms mode on the 200k range.
If R1-R4 are colored orange-orange-orange-gold, you should get readings around 33k
If R1-R4 are colored brown-black-yellow-gold, you should get readings around 100k

Suggest you check the solenoid select signals back at the MPU board at U2 on pins 13,12,11,10 during solenoid test mode with your logic probe. They should all pulse low at various times during the solenoid tests.

Sorry, I just re-read the emails above and missed some things.
The 1.4 volts you measured at pin 19 of U2 on the SDB during the solenoid test is not low enough. It should be 0.8V or lower.
Can you ground this pin during the solenoid test and see if any coils activate?

I also gave you wrong pins to ground for manually testing the solenoid select logic lines on the SDB.
Disconnect the J4 connector from the SDB and leave it disconnected. Run a wire from ground to J4 pin 7 on the SDB and leave the wire there making sure it's not touching other pins.
Power the machine on. If any coils lock on (they shouldn't), abort immediately.
In attract mode, if you then *very briefly* jumper another wire from ground to the SDB on:
J4 pin 3 should fire the Treasure Chamber Saucer
J4 pin 4 should fire the Right Pop Bumper
J4 pin 5 should fire the Left Slingshot
J4 pin 6 should fire the Right Slingshot

If solenoids don't activate, then U2 (the 24 pin 74L154 chip) on the SDB is faulty.

If you did this while J4 pin 7 was still connected to ground, then yes the 74L154 is faulty.
Some of the pins I incorrectly asked you to check up top actually indicate U3 and U4 are probably ok (not dead).

Good luck with the replacement of U2.

Great!

Is there an issue with the lamp board or you haven't plugged it in yet?

When you turn the volume all the way down on the sound board, does the buzzing go away?

On the sound board, what happens if you jumper a wire from TP2 (GND) to pin 1 of U8? Grounding this pin eliminates nearly all the logic chips on the audio signal and leaves you with the amplifiers only.

This essentially mutes the volume to the input of the pre-amp (U9).

Strange. If the buzzing goes away by muting the volume at the pre-amp, then the same should happen when you adjust the volume all the way down to the ouput amp (U10).
I think "all the way down" is actually fully clockwise (i.e. opposite to a standard audio volume control).
Does rotating the volume control make any change at all?

Are you getting any pulsing on pin 9 and pin 11 of U11?

For U3 to pulse, it requires U1 and U2 to be working/pulsing.

U1 and U8 are "4049" chips. Try searching for CD4049.
U2 is a 4042 chip. Try searching for CD4042.
These chips are easily available.

If you're getting no pulsing on the output pins of U1 and U2 they should be replaced.

Can you give me the logic probe levels on all 16 pins of the PROM at U3?

Try measuring the voltages on the pins with your multi-meter instead.

??? Pin 16 is the 5V power pin. You've lost power to the chip..

With the backbox switched lamps, there's a common braid wire to all the switch lamps. Do you measure 5.4VDC along that common braid wire?

Are you getting 5.4V at TP1 on the rectifier board? If not does fuse F1 test ok?

With the work you did on the rectifier board, did you replace the terminals in the three connectors?
The blue wire in pin 6 of the J3 connector at the rectifier board carries the 5.4VDC power to the backbox switched lamps. Measure it there and determine if you have an open circuit on this wire to the backbox switched lamp braid.

How about the playfield switched lamps? Any/many working?

If you can, go into lamp test mode and take a photo at the moment the 1/3 of the playfield lamps are on.

Have you replaced any of these lamps that are not coming on in the chance you have a large number blown lamps?

You have a list of the lamps that are out.
If you look at the lamp board schematic, you can trace them to the SCRs (the things that look like transistors) and then back upstream to the four "4514" chips that activate them. You may notice some things.

In the mean time, you can test the SCRs.
If you look at the small and large SCRs, some of them have the letters "C", "A", and "G" showing you how the pins are defined.
Jumper a wire from TP3 on the lamp board to the "G" pin on any SCR, and the lamp connected to it should light.

Try it with a few lamps that aren't working.
If a lamp lights, the problem could be upstream at 4514 chip or connections to the 4514 from the MPU board.
If a lamp doesn't light, then jumper a wire from ground (you can use "TP2 GND" on the lamp board) to the "A" pin on the SCR (for convenience, the larger SCRs have a metal tab/backing which is internally connected to the "A" pin). If the lamp now lights, the SCR is faulty.

The playfield switched lamp sockets have a common braid wire connected to them all at the base where the lamp socket screws to the playfield. This braid is the 5.4V power for the switched lamps.

The terminal on the tip of the lamp sockets where each have an individual color coded wire connects back to the lamp board at the "A" pins of the SCRs.

With switched lamps that aren't working, if you jumper a wire from ground to the colored wire on a lamp socket tip, the lamp should light up.
If it doesn't, the lamp is blown, or the lamp socket has poor (loose/corroded) connections to the lamp, or the 5.4V power on the base of the lamp socket is missing.
If the lamp lights up but not when you grounded the respective "A" pin of the SCR back at the lamp board, then you have a connection issue back at the lamp driver board connector.

Check for cold solder joints where the light connectors are attached to the board. Could also be a bad pin in the connector

If grounding the colored wire at the lamp socket works, but grounding the "A" leg of the associated SCR doesn't, then that simply indicates a connector issue as zacaj mentioned because you've lost continuity from the lamp board to the lamp sockets on the playfield for those non-working lamps.