my T3 has the exact same behaviour now. Did you ever come to a conclusion to what was causing this problem?
Or does anyone else know what was wrong?
Mine was completely quiet when I got it, no sound at all. I upgraded the sound OS as per service bulletin 157 and that made the sound come back but with the possesed sound as in the Youtube video above.
I've verified all ROM's (Music/Speech/Game/Display) by removing them and reading them with a BeeProg and they all seem fine.
When running sound tests all ROMs except U7 sounds correct.
Also when opening the coin door there is some speech, this speech sounds like it is played back at 4x the normal speed. Very odd.