You can mute the scratching sound by holding down the Ctrl or Shift keys while scratching; Ctrl will mute back spins; Shift will mute forward spins.
Tip: When you let go of the record, the Ctrl muting will stop even if you have the button held down. However, Shift will continue the muting until you let go of the button. (If you are playing a song in the backwards direction mode the opposite is true).
How to scratch
How to record a scratch sample
Adjusting release speed