From the first menu, select SETUP, then SYSTEM. The third option on the left is SOUND. You'll see:

Effects Volume (slider)
Music Volume (slider)
Sound Quality High
A3D Off

These are the default settings. To speed things up, you can change some of them. The Effects Volume is essential. Turning it completely off may get you a speed advantage (very slight), but it ruins the game.

Music Volume Off

This one is less essential. It sounds kind of cool, but may be too taxing to your system to play in addition to your other sounds.

Sound Quality Low

Turn this down to use lower quality sounds and decrease the load on the CPU.

A3D Off

Turn this off to avoid using 3D sound. If your computer supports it, you may be able to use it without incurring penalty. Leave it off just to be sure.