First of all, don't worry about the degrees ... it may give someone a full head, but it doesn't make them a better musician, necessarily.
Second ... Relax and take deep cleansing breaths so your voice doesn't sound strained. Don't imbibe any dairy products beforehand so your voice won't wound "milky."
Third ... Be yourself and keep breathing.
I've never had to audition for a singing/guitar gig, but I'm auditioned many times for singing/acting gigs, and this is the best advice I can give.
Break a leg ...