Both audio channels only playing through the right side speaker?

Have an M1 Macbook Air, purchased in January 2022. Running Monterey 12.6.8. First noticed the problem about a month ago and trying to sort it out now.

Symptoms: when playing audio through the built in speakers, audio only plays through the right side speaker. If balance is centered, both channels only play through the right side speaker. If balance is set full to R, a test plays R channel audio through the R speaker, and no L channel audio plays. If set full to L, a test plays no R channel audio, but the L channel plays through the right side speaker.

Headphones operate as expected.

Steps taken to correct so far: System is updated to latest version of Monterey available. Have verified audio settings, both in the Sound preferences panel and Audio MIDI setup. Speakers replaced via Apple Self Repair program. Built in diagnostics show no issues.

Is there some strange setting or cache that can be fixed? Next big step is resetting the system to factory defaults, but I'm in the middle of a work project and not risking that at the moment until it is finished.