After looking up on the actual wikipedia page for motorstorm: pacific rift. Turns out you can add your own playlist via XMB by selecting the PS button at any point and nvigating to your songs. This has officially raised my personal rating of the game from a 9.7 to a perfect 10. That's one out of only 3 perfect 10's i've ever given out.
You can't change songs whilst in game though, nor set it to shuffle, but it's good enough for me.