The internal sound player is only really intended to play Wav files, but happens to play some other things as well due to the APIs it uses.
Any playback happens via external codec DLLs that are either part of Windows or installed by 3rd party software. The MP3 crash you're seeing may be due to a 3rd party codec. If you zip up the crash dump and email it to leo@gpsoft.com.au I can take a look and see if it's our code or external.