Your Mac is capable of both aural and visual wonders. Two system preferences—Sound and Displays—control how those wonders are manifested.
The Sound preference governs the majority of the Mac’s audio capabilities—the sound effects it uses, its audio volume, the audio devices the Mac plays audio through, and the input it uses to receive or record audio. The settings for these variables appear on three tabs: Sound Effects, Output, and Input. Let’s run through them now.
Sound Effects tab
You know those little beeps and boops your Mac makes when it’s unhappy about something—when you’ve tried to close an application while a dialog box is present or when you’ve attempted to click something that makes your Mac irritable? Those are rightly called sound effects, and you can configure them on the Sound preference’s first tab.
If you’d prefer a different alert sound to the one you’re currently hearing, simply select one from the list—it will play when you select it. (Old-time Mac users may favor Sosumi.)
Beneath this list is the 'Play sound effects through' pop-up menu, where you can choose the device that plays the alert sounds. If you’re using an iMac or MacBook without attached speakers or a pair of headphones, 'Internal speakers' will be your only choice. However, if you’ve attached speakers or headphones, they’ll appear in this menu as well.
In the 'Alert volume' slider below this area you can configure the volume of any alerts that play. This volume is separate from the Mac’s overall output volume.
The 'Play user interface sound effects' option lets you choose whether to play sounds that accompany certain actions. The most obvious of these is the Empty Trash sound. Disable this option and you won’t hear the trash-flushing sound.
The 'Play feedback when volume is changed' option is for choosing to play (or not) the little bleeps you hear when you adjust the Mac’s volume via the volume keys that appear on the Mac keyboard’s top row (F10, F11, and F12).
The 'Output volume' slider is for adjusting the Mac’s overall audio volume, and it appears regardless of which tab you choose in the Sound preference. This slider serves the same function as the volume keys I just mentioned. The final option in this tab lets you choose whether to show a volume menu in the Mac’s menu bar. With that menu visible, you can adjust your Mac’s volume by clicking the menu and dragging a volume slider up or down.
(Tip: If you hold down the Option key and click this menu you can choose your output and input devices, and open Sound preferences directly from the menu.)
You can attach speakers and headphones to your Mac—via either a USB port or the Mac’s headphone port. When you do, additional choices will appear in the Output tab. Just choose a device from the list that appears in this window, and any sounds that your Mac makes (except sound effects, if you’ve configured a different destination for them in the Sound Effects tab) will play from the device you’ve selected.
Mountain Lion supports something called AirPlay, which is a scheme for sending video and audio over a local network to AirPlay-compatible devices such as Apple’s Apple TV set-top box. If you have such an AirPlay-compatible device, it will appear in the list of devices you can play audio through.
Below the list of output destinations is a Balance slider for adjusting how much sound comes through speaker or headphone channels. Drag this slider all the way to the right, and only the right speaker or headphone will sound. Drag it to the left, and you'll hear the left channel from the left speaker or headphone. This adjustment option is handy if you can’t plant yourself directly between two speakers and need to pump up the volume on one of them to compensate.
Your Mac can record audio, too. You’ll find options for doing so in QuickTime Player as well as in GarageBand. If you have a MacBook or an iMac, you’ll see that 'Internal microphone' appears in the list of available input devices in the Input tab. Attach a microphone, headset, or audio interface to your Mac, and it too will appear in the input list. Just choose the input you want to use.
The 'Input volume' slider that appears below this list controls the gain (input level) of the device you’re using—in some cases. I add this caveat because you can't control all input devices with this slider. A Mac’s internal microphone will use the slider; but if you plug into your Mac a USB microphone that carries a Gain knob, and select it in this tab, you may find that the volume slider disappears, meaning that the system won’t work with that microphone.
Regardless of whether this slider is active, the 'Input level' meter should tell you approximately how high your gain is. If you find that it constantly peaks at the top end of the scale, your gain is too high. Lower the gain by using the gain slider or a device’s gain control, or back away from the microphone.
If you’re using the Mac’s built-in microphone, you’ll spy a 'Use ambient noise reduction' option below the 'Input level' meter. This feature, which is built into the Mac OS, attempts to filter out constant sounds like air conditioners and street noise. It can do very little with sudden loud noises, however.