Freeze Track
If your computer is slow or audio is starting to gap or stutter because of a lot of effects or time stretching, you can freeze a track to reduce the amount of processing the track needs.  After freezing a track, the software uses a temporary WAV file that includes all effects and audio.

You cannot edit the effects or sounds on a track until you Unfreeze the track. ( Dragging a new sound on to a track will unfreeze a track ). 

If you want to lock a sound, go here.

To freeze a track, go to the Track Details Tab, select the track you'd like to freeze and then click the Freeze button.