Ever get that problem with a MacBook Pro when you plug headphones into the headphone socket and when you take them out the built-in speaker volume is disabled?
Sound emanates on start up but even once boot up is complete, System Preferences shows a greyed out box for volume. Until you plug the headphones back in again.
Solution in this instance:
1. Think logically, study symptoms in relation to the whole
2. Observe processes without panic
3. Check user forums and apple support
4. Inevitably try the Command-Alt-P-R but to no avail
5. Skoosh WD40 in the headphone socket and wiggle a stick in it
A bit got stuck in the socket when the headphones were in. Agitation and lubrication managed to unstick it.
Use USB headphones in future