I have had two Black Knights with similar issues and have resoldered all headers on the speech , sound board, mpu and driver board, power supply. As well as the socketed roms, pia, and processors. Maybe a little excess, also ran them in attract mode for just north of 24 hours playing at random times. No issues.

Does it ever play the wrong sounds? If so check the connections to the driver board. It could be misinterpreting a sound combination signal as 'silence'. Unlikely but possible.

If that's not the case perhaps you need a new amplifier chip, or perhaps the connections to the volume pot are bad, maybe the pot itself..

You are correct that 5 and 6 dont matter. Here's an excerpt from the "11a-sounds" documentation. The sound table that follows is for a Black Knight:

ABOUT THE SOUNDS YOU WILL GET
==============================================================================================

Some games use exactly the same sound cards as others, so when using this ROM it is possible
for you to hear sounds from OTHER games that you would not normally hear when playing your
game.

I have compiled a numbered list (decimal and binary) of sounds that Black Knight produces
below:

| PB
| NN 6543210 Sound Name
| -- ------- ----------
| 01 - 0000001 Tilt
| 02 - 0000010 Long Drain
| 03 - 0000011 Stunted echo
| 04 - 0000100 Short Laser
| 05 - 0000101 Boing - Points
| 06 - 0000110 Fast Saucer
| 07 - 0000111 Arrow hitting Target
| 08 - 0001000 Extremely Long evaporation
| 09 - 0001001 Deep Laser
| 10 - 0001010 Alien Evaporation
| 11 - 0001011 Sustained Trot
| 12 - 0001100 Short Raspberry
| 13 - 0001101 Static with radio chirp
| 14 - 0001110 Long Laser
| 15 - 0001111 1-4 Targets
| 16 - 0010000 Short Laser
| 17 - 0010001 Sustained Swirl 30 increments
| 18 - 0010010 Sustained Sword 30 increments
| 19 - 0010011 Sustained Cancel
| 20 - 0010100 Random speech?
| 21 - 0010101 "Game of Black Knight Play"
| 22 - 0010110 "Hahahaha"
| 23 - 0010111 "One enemy Cannot Fight Game of Black Knight"
| 24 - 0011000 "Anyone Fight Challenge Game of Black Knight"
| 25 - 0011001 "Game of Black Knight in"
| 26 - 0011010 "Cannot Play in"
| 27 - 0011011 "End Game of Light Fight Game of Black Knight"
| 28 - 0011100 "Challenge Game of Black Knight"
| 29 - 0011101 "Fight At Night"
| 30 - 0011110 "Fight New Enemy"
| 31 - 0011111 "Fight Enemy"

Notice that 11,17, and 18 are "Sustained" sounds. This means you send them once and they keep
playing until cancelled by sending a 19 (Sustained Cancel). The other cool thing to note about
17 and 18 is that each time you fire that sound, it gets faster, yet 11 does not. There are
30 speeds for each, and the 31st time you fire that sound, it wraps to speed number 1. Again,
send a 19 to the sound card to cancel.

I just made up names for these sounds, but you get the idea. Use this ROM to play each one,
and take note of which ones actually come out of the card. If they are incorrect, use the
binary numbers in the list to map which signalling wires should be actively blipping low
for the sound card. From there you can deduce which signals are not getting through. Check
corresponding Transistors on the driver board and/or pins on PIA4

Finally, If you are getting all five signals TO the sound card, but are still having problems,
the problem is with the sound card itself. Since that card is its own beast, I'll need to
write some test ROMs that can be placed directly on the sound card. But that is for another
day!

Pincoder Diagnostics Inc. Version 2020.06.23.0506

---

Have you checked the power going into the board? Perhaps it goes a bit low when it warms up.

Maybe the CPU chip is crashing.. You could try swapping the CPU chip from the sound board with the one on the MPU board. Just make sure they're identical (6802 vs 6808 etc).