I've always hated these things and think music is a terrible choice as it can only ever reinforce the 'middle of the road' tastes (something I have big problems with in NZ).
But the ringer sound just reduces your brain to mush after a few minutes as well...
Would silence work, with the occasional 'you're _n_th in the queue' update? I think that's my best choice
I like music. You can put the phone down and still hear the constant "still on hold" sound, when it stops you know it's being answered.
Given to me it's a tool for that purpose, the actual music doesn't really matter as long as it's constant and lound enough. Probably best not to be any music I like, I'm never waiting on hold for good reasons so best not to associate songs I like with a bad experience.