Take this next bit of information with a pinch of salt but if you are willing to believe a forum comment on Reddit, it looks like the Windows Phone 8 update, Apollo+, will bring Gif and Wav support to the platform.
We haven't heard much officially from Microsoft but Apollo+ will be among the first updates to the Windows Phone 8 platform. The update is expected to arrive in Q1 of 2013. We should note that SoH_ArBiTeR did state that he/she works for Microsoft but we can't verify this information.
Posted below is the comment from Reddit that states the following information:
Now, obviously, we don't know the background of the individual but if you do follow his/her post history, they do appear to be quite knowledgeable on the subject. Only time will tell if the information is accurate, but we hope it's true.
With that being said, look for Apollo + to arrive in Q1 and hopefully it will bring Gif and Wav support along with a few other enhancements and tweaks that Microsoft is currently developing.
Thanks for the tip Sam!