Q We have several classrooms equipped with Macs of different ages. Although most students are careful to avoid the consequent opprobium, sometimes one lets rip with their latest music download. What is the most reliable way of muting their internal speakers without disabling headphone output?
A Unfortunately Apple has changed the sound output hardware and software considerably over different models of Mac. Although OS X makes it transparent to the user, this makes it hard to come up with a simple solution that does not require hardware mutilation such as cutting the input to the internal loudspeaker.
Neither is sound output to internal speakers and the headphone socket independently controllable in software in most models. Although you could write an AppleScript to mute all sound output, it would be too easy for a user to turn the volume up again, and they would have to do so in order to hear anything over headphones. Macs with a separate line out socket, like the Mac Pro, normally have individual control over that, as they do over digital output, USB audio devices, and outputs from third party audio cards.
One hardware manoeuvre that should be worth trying is to attach a headphone through-plug to the headphone socket. This is a simple plug with a stereo headphone jack on one side, and a stereo headphone socket on the other. Once inserted, this should mute all output to the internal speakers, but allow users to use headphones normally, with their normal software volume controls.
The only snag with this is that some Macs still emit their startup sound through their internal speaker; if they proved to be a residual problem then it might be worth disconnecting their internal speakers, but you would be deprived of startup audio diagnostics, which are valuable when resetting the PRAM, for example.
Updated from the original, which was first published in MacUser volume 25 issue 21, 2008.