It pumps up the volume

AMP

This two-wletter device boosts electrical signals in audio systems—think guitars or speakers. Short for "amplifier," it’s often found in abbreviations.

The clue "It pumps up the volume" plays on the double meaning of "pump up." Literally, it suggests something that increases sound, like an amplifier—which is often shortened to "AMP." The word "AMP" also fits because it’s a unit of electrical current, reinforcing the idea of boosting power or volume in audio systems.