A custom engine sound can be made on the Clark just get a wave file of the sounds. It might be easier for you to adapt a set up from an existing vehicle ( I know there are 6 x 6 boards out there ) due the Clark controlling 2 motors for steering and motoring though. Steering is obviously by servo, with 2 motors controlling the 4 drive wheels and the extra shaft for the front wheels that might be a tricky set up.
Now if your talking a half track the Clark is a board that suits that application supremely well.