Sound issue

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

Reset NVRAM on your Mac - Apple Support

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

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

Reset the SMC of your Mac - Apple Support

It's not available as a separate download. You would have to reinstall Mojave. What makes you think you need it?