How to Change Snow Leopard’s Sound Preferences
Mac OS X Snow Leopard offers Sound settings that you can control. You access them by clicking the gear icon on the Dock and the clicking Sound. To set the overall system audio volume, drag the Output Volume slider. To mute all sound from your Mac, select the Mute check box. Selecting the Show Volume in Menu Bar check box displays a convenient volume slider menu bar icon.
The three tabs here are
Sound Effects: From this panel, you can choose the system alert sound and the volume for alerts. You can also choose to mute application and Finder menu sound effects, as well as toggle the sample sound effect when the volume keys are pressed on your Mac’s keyboard (or from the Volume slider). If you use Front Row with your Mac, you can enable or disable the Front Row audio effects from this pane.
Output: Use these settings to choose which audio controller your Mac uses for playing sound. Unless you’ve installed additional audio hardware, this remains set to Built-in Audio or Internal Speakers. You can adjust the balance between the left and right channels for the selected output controller.
Input: These settings allow you to specify an input source. Unless your Mac includes a Line In input source, leave this set to your Internal Microphone. Drag the Input Volume slider to increase or decrease the input signal volume; the input level display provides you with real-time sound levels. If your Mac is in a location with a large amount of background noise, click the Use Ambient Noise Reduction check box to improve the quality of your audio input.