Each Star Trek show has its own theme music, played during the opening credits. Have any of these theme songs appeared in-universe, e.g. as music someone is listening to or as music a character is singing or playing on an instrument? Do these themes exist in-universe, e.g. as an anthem for the federation?
TVTropes has a page about this: Theme Tune Cameo. It lists several Star Trek examples. For example, here's the DS9 theme being played in Quark's bar, in the episode "Sanctuary":