iMac won't output sound to projector

[Ferrari353]

I have a 27" iMac (I think it's the most current version) and it shows video to the projector, but it won't output sound to the projector. Sound comes out of the computer fine just using the computer speakers, and sound works if I connect it to a different computer, but this one computer just will not output sound. I can't figure out what the problem is. Does anybody have any troubleshooting tips or ideas to fix it?

 

[fullojellybeans]

Did you use an 1/8" audio plug to transfer audio from the iMac to said projector?

 

[NikFinn]

Are you using a method which transmits sound as well as video? Or are you using a dedicated audio cable?

When it's plugged in, have you checked system preferences -> sound -> output? There should be an output option there if it's plugged in.

 

[Ferrari353]

Well it's actually my Computer Science teacher's computer and the cable is a VGA cable with a 3.5mm headphone jack on each side and then it plugs into the wall and then I guess it connects to the projector on the ceiling. We checked the output options and the only option was for the headphone port which is where it's coming from.
Also, what's 1/8"?