I can see how the audio sub would be problematic, but if it were solely used for original music then i can't see it going wry. Then again one of the appeals of it would be the sharing of rarities, but like Matt said a large can or worms could very easily be opened.
Maybe this is being a bit silly but i really hate reading about events in other countries when i'm stuck here in Australia. Take that ridiculous lowercase-sound mailing list. All there ever is now are things like 'Hey guys, i've just opened an exhibition in Sacramento - come along if you're around', whereas it (supposedly) used to be a discussion forum about the 'music of Morton Feldman, the writings of Samuel Beckett and the canvases of Mark Rothko'. Don't really know where i'm going with this, all i'm saying is that sometimes events forums can be a little alienating to overseas readers (the bit i hate most about the Wire, reading all the great things that i COULD have gone to see if i were European like all you guys). I wouldn't normally say this because obviously i could cure this ailment and just not look at it, but if you are going to have to choose one of them and not the others, then i think the Audio would be the way to go with certain limitation that can be discussed, the calender as you said appearing to already be in use.
But the Track IDs, i don't know, do we even need that? I can't say i've seen someone requesting a track ID so far, but i suppose it wouldn't hurt, it would encourage it, which is always a good thing