Sound Gone on Macbook Pro Retina 15" late 2014 running MacOS Big Sur 11.3 Beta
Did you try to run the Mac in Safe Mode? Were the results the same?
What about setting up a new user profile (make sure its an admin)? Is the sound still not working with the new user?
If safe mode and a new user are not showing any results that are different from what you currently experience then do the following:

Run Diagnostics Test
Reset the PRAM


Option-Command-P-R: Reset NVRAM or PRAM. If your Mac is using a firmware password, it ignores this key combination or starts up from macOS Recovery.


Reset the System Management Controller (SMC)

The last resort should be to contact Apple Support for any further testing and then they can determine if repair is necessary. Before calling Apple Support, make sure you have an external hard drive (only for your Mac) and back it up so that you can secure your contents and files safely before repairs are done.