T3/whitestar sound issues

Hi,

I've got some strange sound issues on a Terminator 3. When I bought the game it had no sound at all. I then reflashed U8 to sound OS8 by burning a new U7 ROM as per the instructions in stern service bulletin 157.
Now I have sound but it is not quite right. It sounds possessed. Another user, elfman, has posted a video several years ago with the excact same problem, but I've not got any answer from him. Video is linked below.
I know that no boards has been swapped in this game and I've removed and verified all ROMs in a ROM reader/burner.
In test mode all ROMs sounds correct except U7. I've measured on all pins on U7 with an oscilloscope and it seems that all lines are toggling.
(The fact that the sound OS upgrade seem to have worked, 8 flashes at boot, would also suggest that all address and data lines are correct to U7).
I also measured all address and datalines to one of the other sound ROMs and that seemed fine.
I tried remove all sound ROMs except U7 as a test and the sound is still garbage in test mode.
Can anyone confirm that a whitestar board should sound fine in test mode (U7 test only of course) with only sound ROM U7 connected? This would help further investigation.
Any other suggestions?

This T3 has the newer style Whitestar board, 520-5300-00 REV A. As I've read, I think T3 might have been released with both types of CPU board.
This game has been working, I know the guy that owned it in the past, so I'm quite sure it is possible to get it working... somehow.

For some reason the Sound OS would not boot when starting the game today. I redid the service bulletin 157 and then also burned a new U7. Now the sound is okey. I was quite sure I already tried with a new U7, so I'm not sure if it really was U7 or some other bad socket that caused the problem.

Now another question: When setting the sound volume to less then 5 there is a quite noticable digital scratchy sound. Above 5 is fine but I think that is a bit to loud. Can anyone confirm that a T3 (or other Stern with 520-5300-00 REV A) sounds a bit scratchy at low volumes? I've checked the resistors according to service bulletin 133 and they are correct.

The problem came back a couple of weeks ago after it had been working fine for the last 2 years. This time the sound OS would not boot (no blink on sound LED) and the service bulletin 157 would not fix it.
I found out that when cooling the flash memory, U8 - AT49BV1614, it would boot but not work properly. So I ordered a NOS AT49BV1614 flash from eBay and replaced it and redid the service bulletin 157. Now the sound works correct again.
The problem with the scratchy sound was a faulty U26, TDA1543.