With a SoundzAmazing (
review over here) you can produce pre-recorded sounds from an empty hand, which you can use to (for example) "play" a CD using your fingertips or make an instrument continue to make noise after you take your hands off it (best done with an unamplified instrument, of course, or else it'll come across as simply being a trick by the person on your mixing desk).
Combine that effect with a force and you can play the song that a participant "thought of". Or combine with a force or a peek and you can play the song they picked by reaching into a bag of CDs, or the track they pick from an MP3 player, or whatever. Or combine with one of several varieties of "magic bag" and take song 'requests', written on index cards in sealed envelopes, from participants, and then reach into the bag and when you pull out each one the envelope starts making the sound of the song written inside it,
before you open it and reveal what was written.
Instruments more your thing? Should be easy to levitate an ocarina or a mouth organ, but I get the impression you're looking at something bigger: somebody with a greater experience than I of levitation can probably advise.