Press one of the following buttons when you score a touchdown to make sounds while the player is celebrating.

Blow horn: Press (B).
Whistle: Press (A).
"Pow!": Press (R).
Grunt: Press (L).

After you score, press various buttons to hear celebration noises like Madden's patented "boom!"

After the play ends, run towards anybody near you and press (L). You should knock him down. Note: This does not always work.