I had a similar problem on my MacBook Air (13-inch, Early 2015) - no audio would play, except I could change the Built-in Output’s volume. I tried a variety of fixes (resetting SMC, upgrading to Big Sur, shutting down and restarting), but none of them worked. Then I pressed Shift + Ctrl + Option + Power (usually the keybinding for an SMC reset) '''while my computer was still on''' - it suddenly shut down. I pressed power again, and when it turned on the speakers were working again.
Hope this helps anyone with a similar problem!