This is certainly possible through the drop down box on the tool bar (top right of the screen). The drop down box defaults to "Volume," so just click the drop down box and select "Rate."
Next, just adjust the "pivot points" (the horizontal line that is on the sound file - the default is at 100%). Raising the pivot points will speed the sound up, lowering it will slow it down. Currently, when adjusting the rate, the pitch changes as well. In Mixcraft, you can change the playback speed without changing the pitch.
- Click on the sound you want to speed up or slow down.
- Select "Rate" from the drop down box on the tool bar.
- Raise the horizontal line on the sound to speed it up, lower it to slow it down.