So the question came up for each of the characters - can they sing?
- They don't easily fit into a given ability score or skill. Someone who's a great athlete could still have never learned how to swim. Someone who's a great singer may not know how to whistle.
- They don't work as skills that the characters can invest in, because many if not most people in the setting can do these things at some basic level; some people are hopeless at it, or never learned; and some people are naturally great at it. The distribution is two-tailed with a fat middle; it's not something like lockpicking where most people are bad and only a few are trained.
Another kind of solution, more compatible with the rules-light old-school ethos, is to key singing ability to the closest available stat, in this case Charisma, and roll an off-the-cuff check. But something about this solution also doesn't satisfy. It assumes that someone who is a great singer will also be a great leader and vice versa, but these things don't always go together.
In play, when the party's singing ability was called for, I just had them each roll d6 and note the result on their sheet:
1 = hopeless at it
2 = not good
3-4 = average
5 = good at it
6 = great at it
This system then got used in actual play to determine further uses of competences such as whistling (don't ask) and riding.
More recently, I've produced a character sheet with a list of short words, using an even simpler system. For each competence, you roll the die, circle the word if you got a "6" - meaning you're especially good - and X it out if you got a "1" - meaning you're especially bad.
So far, the list is: Ride, Swim, Sing, Dance, Play (instrument), Gamble. In practice, some of the competences do relate to ability scores if people are trying to do something exceptional or contested, but mostly it's a case of whether or not people can do the activity at a basic level.
Any more ideas for the list?