Oh, I was confused by the multiple actors in the clip XDI was reminded of Avery Brooks, another actor that has portrayed a starfleet captain and also sings.
More impressive: he wrote the song.
Mary Elizabeth Winstead singing in Tarantino's Death Proof.
Besides acting she also sings under the name "Got a Girl".
You might be thinking of Boogie Nights. Brown Bunny is acknowledged by Sevigny and Gallo to have been real.
It was an incredible movie, I would absolutely recommend watching it. Espeically if you like any of the actors involved in it (but also just go as blind as possible into the movie if you don't know much)
Them dating was only a rumor. But the fact is he was also the director of this movie, which makes things worse.The actor she blows is her real-life boyfriend if I recall correctly
For example Scott Bakula sang and played the guitar for real in Quantum Leap: