When pitch blocks are moved by dragging an audible tone will be generated corresponding to the fundamental pitch of the audio at the mouse pointer position. This is mixed in with any other audio that may be playing at the time.
This function — "Auto-play when editing" — can be turned off in the Preferences control panel.
The replay level of the tuning preview can be adjusted using the selector drop-down in the transport area of the main display, as shown below. The master volume control below this affects whichever parameter is selected in the drop-down (master volume or tuning preview).