Hmmmm....
Bardic Music doesn't require ANY action aside from pulling out your instrument, if you're using one.
You just start singing, playing, or whatever. Consider it a free action. You can fight, and do anything aside from activating certain magic items and casting spells while using Bardic music... so it is one of those skills that you can do WHILE doing other actions.
For example, a Jump check doesn't require an additional action... it counts as part of your move action.
As far as the DC for hearing the bard... somewhere in the DMG there is a nifty table for listen DCs for certain events. I'd use that.