Momentary in-line volume control
Pass your request up the chain of command to the radio people.  Fiddling may be a way to fix it but having a wire come loose later on a DIY modification will likely be frowned upon (or fail some insurance clause) .

An inline resistor of similar power rating to the speaker of same or up to a few times the speaker impedance in resistance value will likely control the output sound level at minimal loss of fidelity.  You could then have the foot switch operate to short the resistor with a push to make-push to break type switch.  A projected switch failure would leave it in one state but at least you would have some volume.

Modern radios may have clever tricks for remote volume, mute, squelch and road noise inputs for volume control that may be more suitable.

If it uses a very limited number of tones, active notch filters in the guts of the radio may be an option but it is likely to be in a useful part of the audio spectrum and degrade speech quality if attenuated.