This past week in my regular Fall of Magic session, we had a pretty stunning moment where a long-running villain, a siren who has it in for our party, confronted another character with an ominous promise about trouble coming for the party and everything changing. I described him as singing an eerie song as he walked away, clearly performing magic through his song, and one of our players thought about it and said "I know what he's singing," and spontaneously sang us Nina Simone's "Tomorrow Is My Turn."
Given our history with that character, it was a startlingly perfect song — it references how no one knows the future (when this character does, because he's seized control of an Oracle), and how we can try to erase the past, but "It's too late to regret, what is gone will be no more." (Which could be read as a reminder that the party took his lover from him and hid her in memories, where she'll never be found, but he's forging onward anyway.) And it repeatedly comes back to what sounds like a threat: "Tomorrow is my turn," which sounds like foreshadowing for his next move.
We've had a lot of fun in this game with singing to each other — when my character was badly injured, the party found her hiding in darkness and recovering, singing Bill Caddick's "John O'Dreams"
to herself as a comfort lullaby. A bard battle between that siren and our bard had both players writing and bringing songs to the game, with very different tones — hers an upbeat singalong about life on our ship on the Sea of Salt, his a dark, threatening warning about the Magus leading us into peril. We've gotten to the point where we encourage each other to sing, and talk a lot about how music could shape specific scenes.
I've run across RPGs where music was a mechanic or a theme, but this is the first game I've been in where singing was a regular feature and storytelling tool solely because of the people involved. I'd be curious to hear about other people's experiences with incorporating real performance or recorded music into their games to assist in telling the story, especially for conveying emotions.