I updated my macOS to 11.2.3 but my sound stopped working

Have you done a PRAM reset, CMD+Option+p+r...

http://support.apple.com/kb/HT1379

In fact, do 3 in a row, takes a bit of time.

Intel-based Macs: Resetting the System Management Controller (SMC)...

http://support.apple.com/kb/HT3964

Reinstall the OS.

Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold ...

Command (⌘)-R

Installs the latest macOS that was installed on your Mac.