Audio playback slow / stopping despite green bar NEED audio playback smooth

I have 60fps footage in a 15 second compostion which I cut and applied basic effects too. The playback is a little choppy on quarter resoluition but that's fine. What I can't handle is it won't play the Mp3 128kbps file smoothly. As happens when rendering (in the composition not for export) it either stops (if "mute audio when preview is not real time" option is checked in previews) OR in my case, drops into slow bass play back - not sure what you call it.

WHY when my bar is green is this persisting? I need to check the timing of the music and the footage has rendered. Audio is unrelated to fps so it can't be that?