hold shift and click in the timeline, it would loop the audio from the one frame on loop as long as you held shift
It's slightly harder than you described. In CS3 there was a hidden feature where if you clicked and waited on a frame the contextual menu would appear, even if it wasn't a right-click. The shift key doesn't affect that.
But, if you click and drag, as if you're scrubbing, to get the sound to start playing, then the shift key does loop the current frame sound. Or you can click in the timeline frame numbers area, not actually on a frame's cell, and then the trick works right away.
It seems not to be there in Animate. You could request the feature to come back, here:
Feature Request/Bug Report Form