|File names are of the form filename.extension where the extension tells what type of file it is. I believe iphones record in mp4 format but you can check the file you received and see for sure. If so it'll look like filename.mp4.|
Whatever you're using to play the video is probably not completely compatible with mp4. You might try something like VLC player. It's free and should be easy to use. Here's one download site:
There are 32 bit and 64 bit versions there so you should use whichever matches your OS--windows 7 32 bit or windows 7 64 bit.
Even if it's not a mp4 file there's a pretty good chance VLC will be able to play it anyway.
(The above assumes of course that the audio on your computer is already working properly.)