No its not possible short of turning your volume down. As annoying as it is (and its damn annoying I know) a commander that is doing that is actually very good. He is manually spotting every unit outside the UAV radius. Thereby assisting every ally elsewhere on the map.
I used to hate it but realize now that you simply have to put up with it because its what a good commander does. That said, if he isnt spotting different units and only spotting 1 unit repeatedly then that is awful and you should mutiny his sorry hide. However, dont mutiny good commanders simply because the sound of:
enemy infantry spotted
enemy infantry spotted
enemy infantry spotted
enemy infantry spotted
enemy infantry spotted
is annoying.