Volume levels of the music and sounds can be controlled via the Options → Sound page.
All volume levels are relative to your devices volume settings. If you wish to fine tune relative volume levels between music and sound effects then you can use the sound control page.
iPhone and iPads will continue to play any music you might be listening to while playing Trench Racer. If you wish to listen to your own music, set this volume level to zero to avoid menu music playing alongside your own music.