I know what your talking about, the title theme with the xylaphones:
BONG-BONG BONG -BONG BA BONG BA-BONG…
It can also be heard as the main theme in the Terence Malick film Badlands (1973) Starring Martin Sheen and Sissy Spacek, also about a young couple on the run like Clarence and Alabama (it was probably one of the inspirations for the film as well).
Im pretty sure the name of the song is “Musica Poetica”. Or it might have a French title.
You might wanna check the end credits on the True Romance DVD or VHS to be sure though.
