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!
Open up your VirtualDub and load your desired videofile you want changes to be made to.
Changing speed of the video itself is the easiest part to do, go to "Video" -> "Frame rate....".
Now if you want to speed up your whole movie by a factor of 2, enter under "Source rate adjustment" the option "Change frame rate to (fps):" the double of the current frame rate, you can see it on the line above. So example if the current fps is 25, change the frame rate to 50 fps.
Now if you want to slow down your video to half it's speed, you simply divide the current fps by 2, so if the current fps is 25, you'll have to change it 12.5.
As you may notice "Changing the frame rate will cause audio/video desynchronization", we'll fix it in the next part.
This part is a little more trickier, first up go to "Audio" and click "Full processing mode", go back to "Audio" and click "Use advanced filtering". Then go once again to "Audio" and click "Filters...".
You'll need to add three filters, do so by clicking "Add" and add the following filters in this order "input", "stretch" and "output", and close the Add Audio Filter window. Your audio filters should look like this.
Now select the "Stretch" filter and click "Configure". This is where the audio magic happens.
If you doubled your video speed you simply change the one in this box to 0.5, because since it's called stretch, and if you were to double it, you would stretch the audio double as long. In other words, you want to shrink it to half its length.
If you halved the video speed you change the one to 2.
Now go on and select your desired compression, save your new avi and check your result!
Change frame rate to = (original frame rate) x 2
Stretch audio = 0.5
Change frame rate to = (original frame rate) / 2
Stretch audio = 2
So basically it all comes down to simple math, do know when you stretch too long your audio will be distorted, and your image will be blurry. There are other filters and tools to reduce all this, but you can never achieve the same result that you would get from a high fps source.
The sample comes from a Creative Commons Attribution, check it out!