To have full control over sound in the device open the terminal and type "alsamixer -c 0"

Navigate through the switches with left/right, increase/decrease values with Up/Down, toggle mute with m, switch between input and output with Tab and exit with ESC.

You'll have the best chance with sliders named like "PCM", "Master", "Speakers" and mute switches controls containing "Boost" in their names. Haven't experimented with the controls on the N900 myself.