With the set I have the left hand button in the row below the screen controls volume.
You can set volume differently for radio/CD, satnav and traffic news, so there are 3 settings available.
To activate this, the normal high/low regulates the radio/CD. To modify and set the other 2 modalities, when it is speaking in either of them, press the volume button and set the level using the volume control knob.
Even if your COMAND does not look like mine, there should be similar functionality. It ought to be explained somewhere in that large volume they call a handbook.
Basically the volume seems to increase/decrease when at different speeds...although it seems somewhat erratic. I know certain stereos have this feature for when on the motorway etc...but wasn't sure how I might cure things...