Excellent, thanks for taking care of this!
Re: his acceptance comment, the bug is actually in wdmaud.drv; wdmaud.sys you could say is ahead of its time, therefore getting nerfed by the 32-bit API limitation. But let's not nitpick, I probably should have made that more clear in my source comments. Since we're not concerned with Windows 7 (WinMM is never used there by UXP browsers), I didn't delve into it too much, but WinMM did appear to be substantially rewritten there. IIRC at some point official Firefox added an option to pick an audio subsystem, so at least in theory WinMM could be selected even with Win7 (however unlikely that is). I'm pretty sure that in that case my fix would simply be superfluous and not cause any problems, but I think it's really up to them to make sure of that as library providers.