When you use the keys on an Apple Keyboard to adjust a Mac's sound volume, by default, you'll hear chirps. These chirps can sometimes be a little louder than you'd like. Say, when you're turning the volume down in a quiet office or adjusting the volume when your Mac is connected to powered speakers. Here are two ways to prevent your Mac from chirping.
Change volume silently.
You can temporarily mute the chirps to change the volume silently by holding down the Shift key as you press the volume keys.
Turn Off the Chirps.
You can turn-off the chirps altogether in System Preferences. Select System Preferences from the Apple menu, and then click the "Sound" icon (in the second row). Next, click to uncheck the checkbox for the setting, "Play feedback when the volume is changed".