Before I ever went to a convention, I went to lots of music festivals. The two aren't that different. By Sunday, both kinds of events are filled with thousands of people who aren't sure if they just want to go home, or if they never want the weekend to end. While you probably won't find a lot of comic book signings at a music festival, you will find music at conventions.
You can tell a lot about a convention by the artists they bring in to play live. Pacific Media Expo, the Asian pop culture convention that takes place in LA every November, is small, but has a cool party vibe to it. They'll feature up-and-comers and people from the Tokyo club scene, like fashion designer/DJ Takuya Angel. Anime Expo is massive and has national appeal. They'll book artists that are extremely popular within the fandom, like AKB48 and Morning Musume. Comic-Con doesn't offer music as part of its programming (a shame, as the only things that might get me to spend all day in the Hall H line would be the prospects of seeing Orbital and Matt Smith perform the Dr. Who theme and a William Shatner concert), but there are Con-related shows and parties held offsite. We caught Voltaire playing a steampunk party this year.
The best fusion of music and cons that I've seen yet was at Road Warrior Weekend. This was a Mad Max fan gathering that took place in California's high desert last November. Once you hit the entrance for the dry lake bed, you could catch the signal for the con's radio station, which played lots of bands from Australia. Inside the camp site, DJs spun one after the other, pumping techno with a post-apocalyptic feel that lent to the convention-meets-Burning Man atmosphere.
More often, though, you'll hear bands covering theme songs at anime cons or pirate tunes at pirate gatherings. It may not always be to my own taste, but I'm glad that it's there. Music shouldn't be separated from con culture.