I just tried this on a couple of my videos and it is not working for me either. In fact no matter what setting I use it previews music only, won't preview original audio.

Enhancements has been out of order in one form or another for me for several weeks.

I suppose if the issue is limited to Audio Swap preview only you could try a 'blended' setting and use the 'Save As' option and check to see if in fact it did blend without loosing your original.