But then I started thinking back to my own family, and how my grandparents would quite often just start a song up by playing it on the piano. An aunt would join in, grabbing another family member to sing bass or tenor depending on what the song called for, and they'd go at it for a while. Granted, many were religious songs, but not all were. Sometimes my grandfather would just pick at the banjo or guitar and sing some of his own songs during family gatherings, and take requests.
Gauging from how many pianos I've seen in the homes of older people, and how often I've read about that kind of casual musical collaboration in fiction set in their time, I'll have to assume it was commonplace. What happened to it? Did the phonograph kill it, or was it the walkman? Maybe the internet is to blame?
It's possible that once most people could afford a record player, the act of recreating music by performing it was demoted to hobby. Even then, though, I don't imagine record libraries were huge, and it's likely that a family or friend group would know most of the words of most popular songs. So a game of musical "pick up" would be easy. If one person in the group didn't know the song, he or she would learn eventually.
I guess audio cassettes made it easier to distribute and own more music, so as tastes diversified, people knew fewer of each other's favorites. That, and production values got higher and music got more complicated... Now there is so much good music available, I'm sincerely shocked when I have similar tastes in music to anyone, anywhere. As a population, we humans have gained a lot of really worthwhile experiences because of all the available music, but... what have we lost as communities?
As I DJ, I became acutely conscious of how fun it could be to experience music with someone who enjoys it in the same way. Participation by dancing is good too, but participation through performance is more so.
Maybe I'd like to sing Auld Lang Syne with my friends, one year. Do people even do that, anymore?