I use a MBP and sometimes forget about the Trackpad commander. If I leave it on, my wrists will graze the trackpad every so often and it either manipulates the rotor or silences VO. I've noticed that VO is starting to bug out a bit more with the latest version of Mojave, just suddenly going silent and not toggling back on after waking my computer up or after playing a Python-based game I made. Gotta keep tabs on it and file an issue with Apple if it keeps up.

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My Mac was just doing this a couple days ago, I went through the typical troubleshooting steps like checking my sound card, and then going to the voiceover utilities and checking what the settings looks like in there. In the end I ended up just shutting down my computer for about a minute and then turning it back on, and that seemed to fix the issue.