If you hear music unintentionaly, say while wlking down the street, that's not breaking the precept.
Regarding the uposata: do you believe in astrology? To quote Sheldon Cooper, astrology tells us that "the sun’s apparent position relative to arbitrarily defined constellations at the time of your birth somehow affects your personality." If astrology is not of your concern, there is no significance to do the uposata strictly by the lunar calendar. Just do it with the usual frequency of uposata, with the same space in between observance days, just that in those days you don't have choir practice.
He turns his mind away from those phenomena, and having done so, inclines his mind to the property of deathlessness: 'This is peace, this is exquisite — the resolution of all fabrications; the relinquishment of all acquisitions; the ending of craving; dispassion; cessation; Unbinding.'
(Jhana Sutta - Thanissaro Bhikkhu translation)