The diegetically ambiguous music is the conjunction of a music that could be diegetic, as it is congruent with the place (popular music) and it sounds as it is in the diegesis (auricularization, mix), but there is no plausible source.
If the music has a function of describing the diegetic action and really emanates from the scene we can talk about emanation music.
Emanation music versus extradiegetic music in The Pale Rider (Eastwood 1985)
Traditional Georgian music as emanation music in Giorgobistve (Otar Iosseliani, 1966). In this film, music is used extradiegetically but using genres that are linked to the scenes and become always an “emanation” of the diegesis. The audience may expect to see the actual characters sing the music in the foreground.
Ambiguity of the music as a narrative resource in The Letter (Willam Wyler, 1940)
Pop music diegetically ambiguous in Mercy
Diegetic joke in High Anxiety