The Oxford English Dictionary is (unusually) more concise:

Folk music: music of popular origin.

Works for me.

"what constitutes your own in this context, especially given that few of us were brought up with this stuff anyway?"

I heard a headline act sing about how she had a row with her boyfriend and he won, so she went outside and snapped the wing mirror off his car. (Slight American accent, acoustic guitar, barre chords.) I think she was singing from her own culture. Give me the idiosyncratic antiquarianism of working from 30-year-old LPs and facsimile broadsides any day.