This happens when you've been in the ruins, possibly mined something or been in a fight, then came back to the outskirts of the caves, do some work or craft something, only to have the ruins music be in place of caves music. Getting back to the surface and back into caves seems to revert the music to caves in cave locations.
1) Find a cave entrance and enter caves. 2) Go to ruins and craft something/mine something/get into combat with something. 3) Go back to some cave biome and craft something/mine something/attack something. 4) Listen as ruins audio is being played in caves biomes. 5) Go back to surface and back into caves again. 6) Craft something/mine something to hear the caves music being played again.