Bonus songs - Successfully complete the indicated task to unlock the corresponding song:

More Than A Feeling: Get 5 Platinum Records for 5 songs.
Over The Rainbow: Get 15 Platinum Records for 15 songs.
What Is Love: Get 10 Platinum Records for 10 songs.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

American Idol (50 points): Win the full American Idol tournament at any difficulty level.
World Idol (80 points): Win the full American Idol tournament at Expert difficulty level.
Golden Ticket (10 points): Pass the auditions phase in single player.
The Great American Songbook (20 points): Sing all the songs on the disc once - full song only.
American Sensation (30 points): Win 20 American Idol single player tournaments.
It Sounds Like a Lot (20 points): Score 1 million points (cumulative in any mode).
It Didn't Take That Long (50 points): Score 10 million points (cumulative in any mode).
Dedication (100 points): Score 50 million points (cumulative in any mode).
Girl's Best Friend (100 points): Get a perfect score throughout a full American Idol tournament.
Headbanger (10 points): Compete offline in a battle Head to Head game.
Premiership (10 points): Compete offline in an Arcade League game.
Just the Two of Us (10 points): Play an offline Co-op Duet.
Grudge Match (10 points): Play an Xbox LIVE Head-to-Head match.
World Tour (10 points): Play an Xbox LIVE American Idol Tournament match.
Global Star (10 points): Play an Xbox LIVE Arcade League match.
Ranked Grudge Match Winner (15 points): Win an Xbox LIVE Head-to-Head ranked match.
Ranked World Tour Winner (15 points): Win an Xbox LIVE American Idol Tournament ranked match.
Ranked Global Star Winner (15 points): Win an Xbox LIVE Arcade League ranked match.
Maestro (175 points): Achieve Diamond Status for every song on the disc.
Megastar (10 points): Achieve Diamond Status for one song.
Hero (50 points): Achieve Diamond Status for ten different songs.
Superstar (25 points): Achieve Diamond Status for five different songs.
Million Seller (50 points): Achieve Platinum Status for every song on the disc.
Legend (75 points): Achieve Diamond Status for twenty different songs.
Inspiration (10 points): Achieve Gold Status for every song on the disc.
The Showman (20 points): Complete a medley and receive Platinum Status.
Why Don't You Marry Him Then (15 points): Achieve "Excellent" comments from Simon three times in a row during an American Idol tournament.
The First of Many (5 points): Achieve Platinum Status for one performance.