I'm pretty sure all of mIRC's audio events use
MME, and there are known bugs in some audio drivers regarding MME support (e.g. DirectSound works fine (used in games, etc.), but MME-related audio doesn't always work right). I imagine "internal beep" also uses MME.
Windows Vista and 7 adds extra complexity to the mix, since I believe MME on Vista/7 is "emulated" through one of the many audio subsystems, thus actually uses DirectSound in some odd way. Not sure about this though, just speculation on my part.
