It will play .wav and .mp3/.wma files indenpendently. I.E. Play a .wav while it plays an .mp3 or .wma. Midi files may also be independent too but who uses those anymore.
Personally I think this isn't appropriate for mIRC as there are other programs that are capable and even meant to have this ability.
Honestly, why shouldn't mIRC let the user decide. I mean this makes sense for default behaivor. But what if the user uses breif .mp3 files for event sounds, but doesn't want it to stop playing .mp3 encoded song he is listening too?
All audio mixing is done by the operating system. Sound hardware doesn't really matter.