The Flex Timer LED unit does not have an adjustable volume control.  The electric buzzer that it uses is simply on or off.

However, there are a couple of solutions to overcome this:

1) If you find the timer's built-in buzzer too loud, you can apply a small piece of tape to the opening of the buzzer to dampen the sound.  While this is a bit hacky, we have a number of customers who have found this to work for them.  

2) The app itself can be used to play audio instead of the LED unit.  This audio plays out of your mobile device's speaker and therefore can be linked to a wired speaker system or to a bluetooth speaker.