Depends on a lot !
What music do you like to sing ?Ususlly young people prefer Cabaret and older people Gondoliers - but that's a generalisation, and as we all know, all generalisatinos are wrong !
Are you auditioning for 'company' or a specific part ? If its 'company', pick any song you think will show off your voice at its best - but remember not to push the boundaries of your range in audition.
What range of voice do you have ? Here are a few ideas for parts and audition pieces -
Marco (Tenor) - Take a Pair of Sparkling Eyes (youngish)
Giuseppe (Baritone) - Rising Early in the Morning (youngish)
Duke of Plaza Toro (Baritone) - In Enterprise of Martial Kind (fast patter)
Don Alhambra (Bass-Baritone) - I Stole the Prince (middle aged)
MC (Tenor) - Wilkommen (any age)
Herr Shultz (High Baritone) - Heiraten (middle aged)
Not that I should bias you one way or the other but, after XX years in theatre, I recently stepped out of my 'comfort zone' and I'm relishing every moment of the new experience.