You can use a Mac for a long time without ever adjusting its audio settings. As with so many things in OS X, the default settings often work just fine with little (if any) intervention on your part: Your iTunes music plays through your headphones, and the built-in microphone works when you need it.
However, as with so many other things in OS X, you can tweak its audio settings--in the Output and Input tabs of the Sound pane of System Preferences--in all sorts of interesting ways if you wish. Doing so may be required, useful, or just fun to mess around with, depending on your individual case. Here's a general guide to some of OS X's more advanced audio-configuration options.
Sound menu and System Preferences
In typical use, the most you'll have to do to your Mac's audio settings is to change the volume and perhaps adjust the gain of the built-in mic in whatever app you're using at the moment. However, if you use additional audio interfaces regularly, you might need to make more adjustments to those settings.
First, you have the standard Sound item on the OS X menubar, next to those for Wi-Fi, date, battery, Bluetooth, and so on. Normally, you can just click on this menubar item to get a slider that lets you change the system volume. But if you hold the Option key before opening this menu, you'll get more, well, options. On the drop-down, you'll see a list of your available input and output interfaces. This list can provide a convenient way to quickly switch from, say, listening through your headphones to playing sounds through a USB interface that connects to a home-theater system.
In addition to this menu, the Sound system preferences pane gives you access to controls that are useful when you have multiple audio devices attached. On the Sound Effects tab, for example, you can choose which device should play system alerts. While you might consider using the Selected Output Device (the default option) for playing sound effects, you might not want to do so if you've connected your Mac to a large home-entertainment system for music out of; in that case, a warning Ping, Pop, or Purr sound could be intrusive. To get around this, you can use the Sound Effects tab to make these sounds play only on the system's internal speakers or headphones, while other audio is piped out the USB port to your home theater.
Audio MIDI Setup
While these features so far can be convenient, they are just the basics of Apple's audio configuration features. Beyond them, you will have to look to a tool in your Utilities folder called Audio MIDI Setup. In this program, you will have far more control over the audio configuration of your system, and not only change audio formats, but also control the output for speakers, assign different channels to different speakers, and combine separate audio devices into one.
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