To remove the audio from the video or anything on editing/adding. Use VirtualDub
http://virtualdub.sourceforge.net/
I think, its just zip file so extract anywhere you can open it, most likely you program files or your document folder.
once you open the program drag and dropthe video into the VirtualDub window or File>Open video file.. , either way is good. There should be a video in there with the first frame/pic showing two of them. Don't worry about the two pic it just input and output views (From left to right, in and out puts).
-- Remember, It must be a .avi or .mpg and does not work with .wmv or .rm/.rmvb or rather windows media/quicktime/realtime files (to work with those you need a .avs script to show it on virtualdub, don't ask me how. go to google and search videohelp.com something like that.) or .mkv, .ogm (to open these file you need virtualdub mod). --
Also another problem might experience is the codec pop-up error. when that come up then you have to download that codec from somewhere in the web, google time if needed. Another is Audio VBR detected, just ignore that and just click ok/close whatever that shows in there, since you just want to remove the audio.
Ok, once your video is shown now go under Audio>No Audio click that. This enable no audio in the video. So thats done.
Then check the video settings to etract the exact copy without audio, so click Video>Direct Stream Copy. So this will be the exact video quality, if you don't click Direct Stream Copy then your video goes a whopping 1gig or even 2gigs, unless you have codec encoder enabled/configured which is under Video>Compression.
Now to save Video with out the Audio. Click File > Save as AVI..
Hope that works...
- my quick typing..
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Removing audio is very convient in some ways, but it still works while playing on stepmania, it automatically slience it and just only plays the video itself. Removing the audio increases the quality of the video, if you know how to re-encode a video.
For other ways of less lagged videos is to re-encode the video in to a small size of 320x240 resolution or at least under the limit 512 width resolution is alright and a good codec of either DivX or even Microsoft MPEG Codec.
The smaller the resolution the better smoothness on the gameplay. Well, for those who have crap or slow computer.
Codecs with h.264/x264 High.Def video does not work I think on stepmania atm. Plus are you crazy to input it into stepmania? Arrows aren't going show if that inputted, my opinion. But either won't work, I think.
Also, check the FPS Frame Per Seconds or Frame Rate if it is under 60fps. It is common knowledge or not really common that stepmania runs at 60fps at most and puting a video over that limit most likely 120fps will basically kill it, or lucky lagged to max. I haven't experienced that so don't take my word 100% correct.
Check the bitrate of the video, sometimes thats the problem... I think under 2048kbps is alright. (this also determines the file size of the video)
If you decide to re-encode I suggest DivX codecs.
I mostly do these stuff on every simfiles I've done and so use to it... Plus lyric timing ahhh all these works are just pains but results to a good or at least a decent simfile with GOODIES.
- back to my simfile work... sigh.
help plz :) 
