Stereo Sound on MacPro laptop

Follow up:

Have you checked the Audio settings in System Settings... > Accessibility?

I do see a setting there that would force a mono signal.

Exactly what model year is your MBP?

Exactly what version of macOS is it running?

Have you installed any third-party audio apps and/or devices?

Perhaps there is some conflict there.

This is the System Settings... > Sound panel from my Tahoe MacBook Air. There is not a "stereo setting" to enable as I have the built-in MacBook Air speakers selected. Stereo would be enable by default. Perhaps if you have selected some other device, the option would appear(?).

If you are not getting stereo output from your built-in speakers, then you might consider have your computer evaluated by Apple or an Apple Authorized Service Provider.

You can make a Genius Bar appointment for hardware service using this link: Make a Genius Bar reservation - Apple Support