You can change app settings to reduce the computational load on your device's processor which will smoothen out playback on devices that are experiencing static or crackling issues.
First, make sure you are using the latest version of
Music Speed Changer.
Open the App Main Menu
Select Settings
Scroll down and select Advanced
Uncheck "Use native audio library", and change the first two settings, the Time stretch sound quality to Low or Efficient and the Processing buffer size to 20480, by tapping on each
Set Time stretch sound quality to low or efficient
Set Processing buffer size to the max value