I don't think there's enough info here. (Or maybe I'm misunderstanding the question.)
It sounds to me like you want the ability to have
anybody with an iPod be able to walk up, plug their iPod in, and just play music from the cab's speakers.
How is your audio set up? Are you just powering speakers from your motherboard's audio-out? Or do you have a separate amplifier of some kind that has the capability for multiple input sources?
Some motherboards will play audio straight through to the speakers from their audio line-in jack, others require software running to enable it. Like I said, not enough info here. That said, this would be the easiest thing to try first, just use a male-to-male 3.5mm cable and plug it in. If it works, you're done.
Regarding iPods: It's been a while since I had an iPod, but I'm pretty sure they don't just "play audio" over USB. (Someone feel free to correct me if I'm wrong, but I don't even know how that'd be possible.) Also, just "hooking up" over USB, I think you'd have to deal with iTunes, which I also believe still does not (at least, not EASILY) support multiple users/libraries.
Easiest thing to do, if you have a separate amp with multiple inputs, would be to get the appropriate cable (male 3.5mm to whatever your amp requires) and just line it in there. That way, any audio player that outputs to 3.5mm cable (which is almost everything in the portable realm) could be plugged in. If you wanted to be fancy and limit it to iPods only via an iPod Dock Connector, you could use something like
**this**. That cable could easily be integrated into a customized dock.