Thanks for the bug report, and for providing some easy repro steps.
This is a bug that was introduced in Studio 1.07.04. It will be fixed in the next release, Studio 1.07.08, due out first thing next week.
If you need a really quick workaround in the mean time, you can avoid this bug by making sure that you do not place mono and stereo sounds on the same audio track. Two ways to achieve this…
(1) Move the mono sounds to their own audio track. Don’t forget about sounds on parameters too.
(2) Convert the mono sounds to stereo. Performance impact should be minimal, because they were getting up-mixed to stereo anyway.
There is an extra complication with workaround (1) I forgot to mention. You need to force the mono track’s output to stereo format, otherwise the bug may still occur. Do this by right clicking on the track’s output meters and selecting “Stereo”.