Q: No sound when recording w/ quicktime

To enable sound recording, click on the little down-pointing arrow next to the big red Record button.

Did you try all the options?  Which one will work depends on your setup.

Something isn't right. You should always see the option for built in.

Restart you your Mac into Safe Boot, then try again.

Enter Safe Boot mode by holding down the Shift key at startup. Safe Boot is quite slow because the OS performs a bunch of cleanup and verification tasks, so give it time. After you're completely logged in, restart normally.