Precision editing in iMovie 10.1.6

Hi. This is a follow up question to this forum post: https://discussions.apple.com/thread/7848388

I am still using El Capitan and iMovie 10.1.6 (yes, I know.)

I need to adjust the background music (not attached to video clips) slightly to sync better with the visuals. When I select the audio track and try to drag it to the left, it only seems to expand the trim on the audio (it is edited from a longer track). I simply want to drag the entire track a few frames earlier (it does not start at the beginning of the film). Thanks.