It's quite easy to change the actual length of a video file in VirtualDub. I'll explain briefly how to make a slow-motion and a sped up version of your video, with matching sound!
VirtualDub on its own sadly doesn't support many popular codecs and formats, but there is a way to open your favorite mp4 or mkv (and many many others). The only thing you need is a plugin called Virtualdub FFMpeg Input Plugin.
Simply download the plugin, copy the contents of the zipfile in your main folder of VirtualDub and (re)launch VirtualDub and you'll be good to go!