OS X Yosemite: Adjust your computer’s sound output settings
You can play sound through your computer’s internal speakers, your display’s speakers (if it has speakers), or through speakers, headphones, or other equipment plugged into your Mac or available wirelessly through AirPlay.
Before you start, open the Output pane of Sound preferences (choose Apple menu > System Preferences, click Sound, then click Output).
Choose a sound output device
From the list of sound output devices, select the one you want to use. All sound output devices available to your Mac are listed, including the computer’s internal speakers, devices plugged into your computer’s sound port (), USB speakers, and AirPlay devices.
For any device plugged into the computer’s sound port, choose Headphones.
Adjust sound output settings
You can adjust balance and volume for sound output with the controls in the preference pane. If you select the “Show volume in menu bar checkbox,” a volume control appears in the menu bar so that you can turn the volume up or down from anywhere. To set a separate volume for alert sounds, click the Sound Effects tab.
Depending on your Mac and the devices you use, you may be able to set other volume options using the Audio MIDI Setup app.
If you use apps that have volume controls (for example, iTunes or iMovie), their settings can only be equal to or less than the computer’s output volume. They do not override the settings you choose in Sound preferences.