A user brings a technician a mobile device that has no sound. What should a technician check for first?
If the app trying to use the speakers is muted
If Bluetooth speakers work
If external speakers work when plugged in
If the volume is muted

A technician a mobile device that has no sound. A  technician should check If the volume is muted first.

Explanation:

Improper audio settings in your computer can also cause your speakers plugged in but no sound. So you should check the following audio settings to fix your sound problem.An on-screen display appears with incall options. Tap the Mute icon. A microphone icon with a diagonal line through it.Just restart your phone as usual to refresh the device.Disable Bluetooth on your phone. Sometimes the Bluetooth feature would result in the speaker problem so here you are able to navigate to Settings> Bluetooth to switch off the feature to settle the problem.The earpiece may be damaged. This is because the dedicated noise canceling mic is damaged. You might have the sound muted or turned down low in the app. Check the media volume if nothing works out.

Malware, not sure what else would fit that description.

The answer is c.  the soft ware is unable to you because it is open for the public to use.