Here is the full list of all 38 Riff Racer - there are Race Your Music achievements.
Stay in the super-sync zone for 25 consecutive seconds
Unlock 20 packs
Drive 10 cars
Reach Driver Level 20
Reach Driver Level 15
Jump more than 8 beats in one manoeuvre
Smash 30 boxes or more without missing one
Drive 100 songs
Reach Driver Level 10
Drive 5 cars
Drift more than 16 beats in one manoeuvre
Drive 50 songs
Race on a song longer than 10 minutes in length
Unlock 10 packs
Reach Driver Level 5
Beat a recommended ghost
Beat a friend's ghost
Drive 25 songs
Unlock 5 packs
Score more than 100,000 on a song under 3 minutes in length
Play a single song 10 times
Play a Classical song
Drive 10 songs
Author 10 tracks
Drive a whole song without hitting a barricade
Play a Jazz/Funk/Soul song
Boost 20 times or more in one song
Play a Hip-Hop/Urban song
Author 5 tracks
Play a Pop Song
Stay 100% in sync for a whole song
Enter the super-sync zone 10 times in under 40 seconds
Play a Rock/Metal song
Play a Dance song
Roll more than 4 beats in one manoeuvre
Author 1 track
Drive ALL cars
Unlock ALL packs