I'm guessing it's probably similar to Android: it's context-specific; with nothing going on, you're controlling your ringer settings. Then you need to wait until music/a game is playing, then you can control the speaker volume. Want to get to the master volume controls? Have fun menu hunting.
That "context aware" concept has been changed for a while now, current versions of Android always have the buttons control the media volume. (If you are in a call, they control the call volume).
