It will more than likely be royalty-free music. A fee will be paid to a music library, but the film or TV producers don't have to pay royalties after that. It saves on a lot of paper work.
Composers often contribute atmospheric music to music libraries. They get a fee for the work, but no royalties after that.
There will more than likely be no music playing to the extras dancing. Films are usually shot with one camera, so the action has to be repeated for the different camera angles. If the music could be heard, it would be a nightmare for the editor as it would be impossible for the actors to say the same lines at precisely the same time as the same bit of music that was playing the last time they spoke their lines.
Tracking down the title won't be easy as this type of music is rarely, if ever, released commercially.