Rhythmic slash notation: allow playback of underlying chords.

Agreed.

Currently we turn off the playback by default because the assumption is, most people don't actually bother entering the full chords when they are writing rhythmic slash notation. - they just enter a dummy note to indicate the rhythm. So you wouldn't to hear that playback. But if you are taking the trouble to enter the full chord, you can simply turn the playback back on, by checking the "Play" box for the notes in the Inspector. Just be sure to select the entire chord, not the single note that ordinarily shows up, otherwise you'll actually just be turning on playback for a single note of the chord. Easiest way to do get the full chord is to make a range selection, then hit the Notes button in the Inspector to limit select to just notes within the selection. *Then* tick the Play box.

Ultimately, what I know a lot of people would like is something to save you the trouble of entering the chords - where MuseScore would automatically come up with appropriate voicings based on accompanying chord symbols. Someday I have little doubt we will see such a feature in MuseScore, but it's a pretty big project and thus far no one has taken it on.