If the sound isn't working on your Mac Mini, it could be the speakers, the software or the Mac's memory that is causing the problem. If you connected a Mac Mini to external speakers in your office or to a TV to do a presentation, the Mac should automatically use the external speakers. If you connected the Mac Mini to an HDTV using its HDMI port, make sure the volume on the TV and the image are working properly. Once these are checked you can check the Mac Mini's software controls. If none of these work, the last thing to do is to reset the Mac's NVAM/PRAM, which is the memory used to store sound settings and other functions on a Mac computer.
Troubleshooting External Speakers
Examine the external speakers to ensure they are plugged in or that the battery is charged. Most speakers have a small light near the volume control to indicate the speakers are turned on.
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Turn the volume control knob on the speakers to the maximum volume.
Unplug the speaker connector from the Mac Mini and plug it back in again. If the speakers make a noise, like a buzz or click while moving the connector in the port, the speakers are most likely operating correctly. If they make no noise at all, there may be an issue with the speakers.
Mac Software Controls
Select "System Preferences" from the Apple menu in the upper left corner of the screen. Select "Sound" in the Hardware section of the System Preferences menu.
Click the "Output" tab and then select the output you want to use, such as the internal speakers, external speakers or HDMI, depending on what is connected to the Mac Mini.
Drag the "Output Volume" slider to the right to increase the volume. Make sure the "Mute" option is not selected.
Turn the Mac Mini off.
Turn the Mac Mini back on and then immediately press the "Command," "Option," "P" and "R" keys all at the same time -- this must be done before the screen becomes gray. Continue to hold the keys until you hear the Mac startup sound twice, then release the keys. Wait for the computer to restart.
If the computer loses sound again after you shut it down, you'll need to take it to an authorized Apple service center for repair, as it's likely that the logic board battery needs to be replaced. This battery keeps PRAM settings, including sound, in memory when the Mac is turned off.