Soundtrack credits - song on a background, actor humming a song etc

Hi.

I was just wondering what are the guidelines of the Soundtrack credits when it comes to… (some examples follow)

- Song is faintly heard on the background (in a bar, from the radio, in a disco, etc). => This means the background of the scene itself, and identifiable song (not film score). 

- The actor is humming (well, or clearly singing) some song. => Maybe in a karaoke, maybe in shower, maybe in a car when the favourite song of the character is heard on the radio…

And before you ask, not all these type of examples are listed in the end (soundtrack) credits. So they can be ”uncredited”.