While you're on an active call, you can adjust the speaker volume by pressing the volume buttons:(louder) and(softer). The buttons are located on the lower edge of your device, just below the JUMP button.
You can also open the Volume Controls pane by pressing the MENU and DONE buttons. Roll the TRACKBALL right to increase volume, left to decrease.
Caution! During a call, the sound level on your Sidekick Phone may vary, depending on how you are routing the sound. Keep the volume at a moderate level to prevent damage to your hearing, and be careful not to switch accidentally to speakerphone while the Sidekick is against your ear