It looks like you've actually got your fill of guitarists but in Tony Stark's house in the movies he has a couple of nice electric guitars so I headcanon that he can play and because of that with the additional knowledge that RDJ can sing, I headcanon that he's got a pretty good voice.

Not to mention rockband is exactly his style but if you've got enough people/aren't interested it's totally okay since my thoughts on his talent are all headcanon.