My mac won't play sound

Without an error report, your first thing is to reboot.

If you see red light in the headphone port, plug the headphone

in and out of the port 5 or 6 times to flip the microswitch inside.

Sound output may be stuck in the digital mode.

Reset PRAM. http://support.apple.com/kb/PH4405

Reset SMC. http://support.apple.com/kb/HT3964

Choose the method for:

"Resetting SMC on portables with a battery you should not remove on your own".

Best.

If the chimes sound at startup, then it's unlikely to be a hardware fault.

Try re-staring in Safe Mode followed by a normal restart. This should resolve any permissions problems.

You can also try re-setting permissions.