The wall of sound Trophy in WRC Powerslide

The "Horn Power" Power-Up takes the form of a trumpet as seen in this image:

 

This power up allows you to fire a ball of sound (similar to a homing missile) from your car which targets the vehicle ahead of you, thus causing its rear end to bounce in the air upon collision. In order to acquire this power up, you'll need to be in a position other than first place. In my experience, second place received this power up quite often so sticking right behind first place is the best option while attempting this trophy.

To acquire this trophy, deploy the "Horn Power" Power-Up 3 times in the same race, if you're having trouble acquiring it 3 times in a single race, 'Pissia' under the 'Acropolis Rally' is the longest track and offers several opportunities to use power ups. Alternatively, you can keep driving over the same power up over and over again until you earn the "Horn Power" power up 3 times. This trophy can be earned in Multiplayer and Singleplayer.