Is it just the volume that is wonky? Does the screen function normally?

I would boot from Disk 1 and run a diagnostic on the volume. Desn’t sound like you are thoroughly screwed, but I would back up files if you can to a thumb drive or to a dvd if it will let you

something similar happened to me a few weeks ago when i unplugged my headphones and yet the sound wasnt coming out through the computer speakers. Then i noticed there was a red light coming from inside the little whole you plug in headphones. apparently sometimes a little button or trigger thing can get stuck and think it still has something plugged in. If this is your case just take a toothpick and stick it in the whole and poke at it gently till the light turns off. btw sorry if that sounds dirty i didnt mean for it too haha. hope it helps