See if this helps:
On the front of the receiver (the part that you wear) is a large black volume dial. This large dial acts like a "cog" and turns a smaller internal volume dial. In a scenario where the volume control of the Set 830 or Set 840 does not appear to be functioning correctly it may be a situation where the dials are out of alignment.
The solution is to realign the dials. To do this follow these steps:
Step 1: Remove the large black dial. (It pops straight off).
Step 2: Turn the volume all the way down by adjusting the small internal dial. (The small dial is grey colored and about the size of a dime).
Step 3: Re-align the large black dial with the "0" just to the RIGHT of the "12 o'clock" position and click it back into place.
Step 4: Test the volume.
Step 5: You're all set.