It's not just that you "can" play your own songs... the whole game is based OFF of your songs. It doesn't come with any "preloaded" music... Just a complete Valve soundtrack that really are just music files like any other on your computer. When you choose your character, it makes you then pick a song from your HDD and you're off. Such an awesome game!

I logged into youtube to upload a song I recorded and this is what I saw:

- youtube_longer_than_15_minutes_lolwut.PNG (4.31 KiB) Viewed 3573 times
LOLWUT? I don't have a clue what this means my time limit for videos is now... >.>' BUT AWESOME!

Anyway, here's the video for my playthrough of Wazgul's Erectin' A River:
*EDIT*
CHECK THIS OUT! WAHOO! I GOT FIRST PLACE ON THE GLOBAL LEADERBOARDS! :3 I knocked this dude power punch out of his first place throne for the last 7 weeks in three days. :3 I'm so happy.
Audiosurf Song Stats (seen at the end of the vid) :3