Lars,
With the current state of MMOS2, the fact that IBM essentially abandoned it around the time of Warp 3, I am not surprised that there are bugs. I certainly would not put any fault in your driver or applet, in fact I applaud your efforts - I'm really impressed with how well your USB audio driver works.
The sad part - there was a time when MMOS2 was near the top of the game - I can recall when the only system that had better multimedia capabilities was the Amiga. Unfortunately today it can be a challenge just to get a sound card to work. But I can't say that I'm all that surprised.
Anyways, thanks for your efforts. I still have an old Warp 3 machine here, it isn't the one that I had from way back when with 2 audio cards, but I might just see if I can get the Warp 3 box I do have going with 2 cards to see if it works the way I remember. I do recall that when I did have 2 cards working it was on an old machine with all ISA ports (it took quite a bit of fiddling with the jumpers and IRQ's to get it to work... that's how I recall it was an ISA based machine). Unfortunately I don't remember specific details of any configurations that might have been needed. 1996 was a long time ago!
Cheers!