If you prefer to play the game without music and/or sound fx, you have options in the setting menu. Tap the gear icon (at the top left of the screen). Locate the "MUSIC" or "SFX" options and tap the arrow buttons next to each option to toggle "On/Off"

These options are also available during the mission. Tap the pause icon at the top left of the screen during a mission to access the setting menu.

Note: You can always turn the music/sound fx back on by returning to the setting menu.