thermostat wire for in ceiling speakers?

Wire is pretty much wire. That said, it's generally preferred to use heavier than 18 gauge for speakers. Did he run the 4 conductor cable to each location? If so you could double up by using 2 conductors together for each of the speaker wires, making the wire equivalent to 14 gauge.

I'd call him out on it. Unless you didn't give him much guidance, that's a pretty stupid mistake.

I agree it'll *probably* work, though

it will work but sound quality might be seriously compromised

it depends what size speakers and amplifier you have: you will be limited by use of undersized wire: if it is an in wall speaker in a bathroom it might be OK: if they are outdoor speakers by the pool: it will never be right

this is a major mistake by a guy who should know better

Assuming an electrician knows anything about the requirements for a speaker system is asking for trouble. If you specify what you want and don't get it, that is one thing. If you don't specify what you want, you get what you get. Seems OP got what he specified: speaker wires. 18 AWG is apparently what the electrician is used to using. Sounds like typical tract home speaker pre-wire to me.
Edited by Colm - 8/6/12 at 3:08pm

OP said he ran 18/4.

The wire is fine for what it is. It is rated CL2 (same as most in-wall speaker cables) which means it is adequate for class 2 circuits, which most speaker cables are. The only noticeable difference may be it is solid wire. Speaker cable is usually stranded. But it really makes no difference for in-wall wire. Stranded wire is better for something that is repeatedly flexed. I doubt you wil be repeatedly flexing your in-wall wire. According to the table on Roger Russels's site. 18 AWG is adequate for a 8 ohm load up to about 32 feet. Double up strands of the 18/4 and it is fine for about double the distance. If the speakers are lower impedance, the distances are reduced.
Edited by Colm - 8/6/12 at 2:56pm

The thermostat wire will work, but unfortunately your electrician did not