Timer started: Perhaps "Go!".
Timer Stopped: When the timer is stopped, for whatever reason during a match, often players continue to shoot because they haven't noticed. An appropriate sound would help prevent this. Also if a big "STOP" was flashed across the screen, it would help those playing with the sound down (i.e. at work or when they should be studying

Almost Timeout: Perhaps a "Peep" sound for each of the final tens of seconds and then the final 10 (i.e. at 30, 20 10, 9, 8, 7, ...). The amount should be configurable on the client. This would certainly add to the excitement for close finishes.
Timeout: Perhaps this could be the "Flag-Capture" cheers/laughing sounds, depending upon whether you won/lost with another sound for a draw (defaulted to "cheers" but configurable).
This enhancement should only require modifications to the client and would, in my opinion, greatly enhance the user experience, especially in matches.
So what do you think?