If you don't already have an unlocked audio, go to the Community Market > Unlockables and filter by audio.
You can choose to buy the cheapest audio if you want or put in a buy order if you don't want to spend as much as what is already for sale (this will take longer).
Once you have your audio, go to Diamond Dynasty mode > My Squad > Customize Team, then equip the audio to any action/event.
Note: I initially tried to equip an unlocked audio in Road to the Show, but the trophy did not unlock. I then went to Diamond Dynasty and tried to equip the same audio to a different action/event and it still did not unlock. I had to buy another audio and equipped it in DD first and the trophy unlocked right away. So do this in DD not RttS