You can use some online free Youtube mp3 converter, that will also have the possibility to convert to mp3. You will find plenty of great free instrument online. However, I am not really sure if DeskScapes supports the mp4 files. As I remember, it only allows you to use WMV or AVI files. So if you try to post an mp4 file, DeskScapes will simply not show it (but if you guys are searching for this now, check this, cause I am not sure if it is stil actual, or they changed it already). But think should not be a problem, after you convert the file to mp4, you just do it again, but from mp4 to WMV. Hope it will be useful for you.