WMV is not a good solution as it would only be able to be viewed my users with MS Windows or even using Internet Explorer. You would have problems with Firefox /Mozilla and Linux / Macintosh visitors.
However, my guess is that its possible that memory_limit is causing you a problem see art82. Tiki has to load the file into system memory in order to then pass it out in a stream to the browser. This is the same for image/file/forum galleries. Its written this way for security reasons.
Id check your memory_limit variable, and if possible increase it as per that document art82