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Forum Moderators: travelin cat
I'm not using an iMac, but that's where the sound controls have been for a long time on every other Mac I've used (tho' it used to be the "Monitors & Sound" control panel...).
The Apple Menu is found via the little Apple icon in the top left corner of the screen. Output is one of the tabs in the Sound control panel.
You can also give them the address of Apple's support site, as it doesn't sound like this is a part of your normal job responsibilities. ;)