HTC Desire :: No Audio On Some MP4 Videos

I use a little application on my computer called iPlayer Downloader. Basically it lets me download BBC iPlayer programmes as either mp3 or mp4 files.However when I copy one of the mp4 files to my Desire it plays but without audio. I've tried other mp4 files - one downloaded direct from YouTube and another via ZamZar and they work (as do the mp3's it produces of radio shows).I assume it's something to do with the audio format, but on the above the only difference on the non-working one is the Stereo format