Hi Granicus_1967,
Thank you for using the Apple Support Communities! We understand your left speaker stops working on your MacBook Pro, and we'd like to help.
It sounds like resetting the NVRAM helps get your speaker working again, but it stops some time after. Do you notice if your speaker stops working again only after using a certain application or software program?
Let's have you boot up into safe mode. It’s a way to start up your Mac so it performs certain checks, repairs, emptying of system cache, and prevents some software from automatically loading: How to use safe mode on your Mac - Apple Support. Once in safe mode, check to see if the same behavior continues with your speaker. Sometimes safe mode can successfully resolve some issues. With this being said, after testing in safe mode, restart and log in as normal to see if that helps resolve the issue.
If the issue remains, test a new user account next, to isolate if this is a user-specific issue, or a system-wide issue. Here's an article on how to set up a new user account to test: Set up users, guests, and groups on Mac - Apple Support.
Keep us posted on the outcome of those steps, if you need continued support.
Regards.