One of the most powerful circles I ever attended was run by a facilitator who began singing the Oh-ma-ma. Slowly everyone added their voice to the choir. Melodies, harmonies, drums... the vibrations were truly awe-inspiring.
In the druid world singing is a big part of worship. You will find music/tunes that that set you off in a daydream, it is when you add to this music a spell. This is when casting a tune comes in to it's own. depending on how much you put into it, it can be very powerful. But be careful what you wish for.