An audio pickup was a listening device.
In 2367, Reginald Barclay created a neural scan interface that communicated his thoughts directly to the USS Enterprise-D's central processing unit. This meant much of his higher brain functions and memory were transferred to the starboard computer core while his body remained in the holodeck.
As a result, Lieutenant Commander Geordi La Forge disconnected the audio and visual pickups inside the observation lounge so that the senior staff could talk privately without being monitored by the ship's computer/Barclay.
Later on the bridge, Captain Jean-Luc Picard gestured Lieutenant Worf to disconnect audio before ordering a security team to Holodeck 3 to disconnect Barclay from the computer. (TNG: "The Nth Degree")