Play Instrument (sage ability)
Play instrument is an amateur-status sage ability describing the character's skill in playing a traditional instrument, typically a mandolin, lute, violin, drum, horn or the pipes. This is considered to be the musician's first instrument, so his or her master should have taught an instrument that is lightweight and easy to maintain and play. Only one instrument may be chosen.
Through the musician's training and from elsewhere, it's understood that the musician has acquired a bevy of songs — ballads and songs for drinking, working, love, sadness — and that he or she is able to perform these competently. Naturally, the musician likely plays these often, with music becoming familiar around the party's camp on a near-daily basis.
Performances
However, once a month, or after a period of at least 30 days, the musician may give a performance, presenting a new song that's been learned and practiced — and the pleasure received from listening to this song gives the audience a feeling of well-being, confidence and clarity of mind. This translates into a +5% experience bonus that lasts for twenty-four hours. Since the musician can decide precisely when to give this performance, presumably this can be prior to an expected battle or in the hours before entering a dungeon. The performance must last half an hour and can be given while waiting on the battlefield (when martial strains would be expected) or — if practical for the instrument being played — while approaching the dungeon or enemy's lair.
A performance may be given in any new venue the character enters, for the patrons there won't have heard the musician play before — thus the "once a month" requirement is met for each stranger. The party may not receive any special benefit from most of these nights, but the new audience will. Importantly, the audience must be ready and appreciative of the would-be performer! A performance cannot be forced on a crowd who happens to be present, but are not interested in listening. Thus an introduction by the proprietor is also required for a performance to be practical.