I think it might still be done by tapping the LEFT of the status bar on the 920. (TomTom keep moving the elements of the status bar around, so it may also depend on what software version you are running)
At least that's what it says in the on-line manual....
How do I change the volume level?
You can change the volume in three ways:
• Tap the bottom left hand area of the Driving View. The last spoken
instruction is repeated and the volume level is shown.
Move the volume slider to adjust the volume.
• In the Main Menu, tap Change preferences, then tap Volume preferences.
Tap Test to check your changes.
• You can set your GO to change the volume automatically depending on
how noisy it is in the car.
I don't know why you can't find it in the preferences menu... I keep an old Go720 on early software, and if I got to Main menu / Change preferences / Volume preferences, the volume slider is there.