I get nervous at auditions, but that's natural and welcomed, really. I'd hate to be blasé about them.
Learn the tunes (note-for-note) go along, be nice, play and then leave. If you get in, fantastic, if not....don't immediately blame your bass playing - the person after you might just be a better fit for the band, might like the same football team as the singer or have gone to the same youth fascist group as the drummer.
Focus on doing your best, not about how it'll turn out.