Sounds like a resource conflict. Try removing any unnecessary hardware and see if the problem persists.

I've had some games where the midi sound went all bad, in my case it was because the MHz of the Pentium was too fast.
at 200MHz there was bad hanging notes, at 166MHz the problem was still there, then around at the 150MHz and 133MHz area, all was well. My friends computer back in the time of 1996 had the exact same problem. We temporary disabled the internal CPU cache to get the old games OK. Slightly newer games then ran too slow! 😀

Thanks for the replies!
I already tried the system without all other add-on cards.
But tomorrow I will also remove the cd-rom and other drives and disable all unused ports on the MB to see if it makes a difference.

If that still fails I will try to lower the multiplier and see if it will change anything (though I had trouble with every game I tried with midi playback).

Thanks for your troubles and I will post my results as soon as possible. 😀

Removing all hardware didn't solve the problem.
Lowering the multiplier so my cpu ran at 100 mhz didn't solve it as well.
I also tried lowering the FSB to 50 mhz instead of 66 mhz, but this didn't work as well.

Any other suggestions are welcome and appreciated. 😀

Test the power supply, sound cards are often one of the few devices making use of the -12V and -5V rails, if the amp is not being powered right it can produce funny results.

I had an AWE64 (Regular) that did this in a P2-266 circa 2006, I never figured out what was causing it in that though as I swapped it with another AWE64 which worked (I assumed the card was broken as it had worked for a while, despite being rusty) but later tried the original card having decided I wanted to fix it only to find the problem was gone. That card worked fine until a few weeks back when a capacitor gave up and the system could no longer detect it.

However, I do recall the Pentium II I was running having a very weak PSU. I also seem to think I may have changed slot when I put the new card in and that workspace was dirty, so it could have been just a bad connection, maybe try another slot too then.

I wanted to try HighTreason's suggestion. Problem is, the system uses an AT PSU (SA131-3400). The power button goes directly in the PSU, so not to the MB.
I have a different AT PSU bit it appears that it doesn't have a direct power-on button like the current psu...
Therefore I am currently unable to test this solution. 🙁