Best Buy has the same deal for Jam Sessions as they (and other retailers) have on some CDs: extra songs. Which is kinda weird for a video game; how pissed would you be if Guitar Hero 2 had special songs you could only play if you bought it from Best Buy (or, god forbid, Gamestop)?
However, for Jam Sessions, the extra songs manifest in the form of a little sticker on the front, not unlike the "Redeem me now!" coupons I've seen on loaves of bread and other groceries. How does this work? Hilariously, you unlock the songs by entering a code in Free Play mode with the directional pad.
For bonus hilarity, what exactly is that code? If you're a gamer, you can guess it. It's even the code you might enter aimlessly in a moment of boredom. (It does nothing on the title screen of Ouendan, trust me.)
Yes, indeed, it's the good ol' Konami Code, ↑↑↓↓←→←→. (Which you might know if you'd searched for it, but since I picked up my copy just now from an actual Best Buy, I had the joy of discovering it serendipitously.)
Surely it works on copies you buy from elsewhere besides Best Buy. Let me know if you bought the game somewhere else and it does.