High-Pitched Announcer
At the Options screen, press Select 40 times.
Snowman
Before you can do this code you have to beat the special stage. After you beat the special stage in all three categories you will have access to the snowboarder "Snowman". Start a new game, then choose the best Alpine board by pressing Up, Circle, and Triangle.
Special Course
To access the special course, beat every course in all categories (best time, best tricks, and most points.)