Change the output volume of your Mac running OS X Yosemite 10.10 if the sound alerts are too loud or too soft, or your music volume is not comfortable and you can't change it from the audio player software. The System Preferences utility contains many of your Mac's settings, including alert and output volume. Furthermore, you can change the sound balance for your speakers from the Sound pane of the System Preferences utility.
Change Sound or Alert Volume
Click on the Apple menu at the top of the screen, select "System Preferences" to launch the utility and then click "Sound" to bring up the Sound pane. If you can't locate the sound icon, type "sound" into the search box on the System Preferences screen.
Select the "Output" tab. Increase or decrease the output volume by moving the "Output volume" slider. Mute the speakers immediately by checking the "Mute" box. Select the "Sound Effects" tab to change the alert volume by moving the "Alert Volume" slider. Check the "Show volume in menu bar" box to display the volume icon in your Mac's menu bar; use it to quickly change the sound volume.
Change Sound Balance
Change the sound balance for your speakers by selecting the "Output" tab and then moving the "Balance" slider. Select the correct sound device from the list if you have several devices attached to your computer. Move the slider to the left to shift the balance to the left channel or to the right to shift it to the right channel.