I think I have solved my own problem. It appears that frame trimming was disabled. I'm not sure if this was the default after my recent installation (would seem unlikely), or if I changed these settings while trying to get frame trimming to work. My problem was solved by un-checking these 2 options under control in the main menu bar:
Slider K-Frame Positioning
Markers K-Frame Positioning
Some background information: Zoom session recordings are extremely optimized MP4 files to minimize the resulting file size. If the video image is static, K frames will only be generated every 36 (thirty six) seconds. If you open the zoom session a bit early for people to join, and have a static welcome screen displayed during that time, you'll get the 36 second intervals for K frames. Trying to trim to the start of the presenters voice, using K frame trimming will get up to 36 seconds of the preceding video and audio, which may be inappropriate, especially if your attendees are rowdy. Even if you mute them in Zoom, which we don't want to do, you'll get a potentially long gap of silence before the presenter is heard in the recording.
This isn't a criticism of SolveigMM, just an illustration how for Zoom session recordings in particular, frame trimming is a critical feature.