Dogma - Alanis Morrisette (as God!)
Art Garfunkel in Carnal Knowledge.
Paul Simon in Annie Hall.
Tom Waits has been in 39 films: https://m.imdb.com/name/nm0001823/filmotype/actor?ref_=m_nmfm_2
I discovered Gogol Bordello in a film called Everything Is Illuminated.
^haha hell yes! i forgot about that one and it's probably the best cameo ever
Follow that Bird - Waylon Jennings being the most badass dude ever when i was a kid
and literally anything with a bad casting of Flea from the Red Hot Chili Peppers. that guy has popped up in so many bad roles: back to the future 2 and 3, the big lebowski, fear and loathing... and a bunch of other movies I'm not thinking of. he's like the go-to casting if you need an ugly/dumb villain... and it cracks me up whenever i see him
Then James Brown made a comeback in a Rocky sequel.
There's Hoagy Carmichael in To Have and Have Not.
The Marx Brothers - A Day at The Races & A Night At the Opera, are just two.
I know Harpo was a musician did any of the other Marx’s play instruments?
I can’t remember which film but there’s a scene where Groucho is playing a nice arch top gibson in a boat on a pond and he stops play and he throws it into the water ( although it turns into a cheaper guitar for the throwing)
I could never tell if it was actually him playing or not
I guess most those guys who started in vaudeville could play or sing