This error means that the Speech-Over media effects cannot be saved to wmv format without first doing a preliminary conversion on the pptx file.
To do the preliminary conversion, click File/Info and click Convert. Save the converted pptx file with a different name. For example, if the original presentation is A.pptx, name the converted presentation A-converted.pptx.
Now you convert A-converted.pptx to a wmv file by doing Save As Windows Media Video (wmv). You can name the wmv file A.wmv.