I'm not at my editing computer so I can't test, but try this. Hold down the Control key and left click and hold down the mouse key at the end of the clip you want to shorten or lengthen. You can now move the end of the clip to the right (to lengthen/slow down) or to the left (to shorten/speed up) the clip. I believe this will work without changing effecting the audio.
If this doesn't work, consult the Help Menu and/or download the program documentation from Sony web site.