I've been playing a 4-member band household for a bit, and in the 5 or so shows they put on, there hasn't been even a single Sim in the bar aside from the mixologists. They still get told it was a great show, but I do find it a bit depressing. Of course, it doesn't help that most gigs were scheduled for 12am, which during the weekday means just about everyone is at work.
Even during the one 8pm gig, the whole town decided to spend the evening elsewhere (the bar wasn't a hotspot). Jamming is decent enough, but it does get old, so I do wish I'd get gig offers a bit more regularly -- and that anyone would actually show up for them.
So, have your bands had proper audiences? Do Sims watching a gig have any special reactions or is it just the usual tap your foot or dance with a buddy routine?