there arnt necesarrily regulations against stereos on carts, just the fact of blasting your stereos on your cart. My team, 1251, had a nice little summer project of cart building, all individually funded from the members. we were a little obsessive on it, since the audio totals about 2200 watts, we have a dvd player and 2 lcd screens , basically, the cart shouldnt be your primary goal, just a nice offseason project since last i remember, you couldnt work on robot parts during that time frame.
but as long as you remember that the team next to you doesnt want to hear your music like their at a concert. tone it down, and stereos will be ok