That callstack does show where the crash happened, but not why it happened. I'm not sure the dump file would reveal that, as the exception record appears to be from the .Net exception handler rather than the BASSmix crash location. But I have an idea what may have caused it (possibly a simultaneous BASS_ChannelSetPosition call on the same mixer in another thread?), and here's a BASSmix update for you to try:
www.un4seen.com/stuff/bassmix.zipPlease let me know if you still get any BASS_Mixer_ChannelGetPosition crashes with that.