Compaq Armada 1500 User Manual page 13

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Volume level of the stereo can be changed one step at a time by
pressing and releasing the volume buttons. The left button adjusts
the volume down and the right button adjusts the volume up.
Volume level can be changed in multiple steps by pressing and
holding either button.
Pressing both buttons at the same time mutes the sound. With the
audio muted, system beeps are still audible unless they've been
turned off using hotkeys or the power icon.
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