I can't solve the same problem in my side.
I would like to play a mp4 file h264 with mediaelement but I can't. I get a mediafailed message -  c00b11d1.
Anyway I can play it with Windows Media Player - i have K-Light codec pack installed.
I use a Vista x64 ultimate - but I tried to use the application on a x86 XP.
Anyway I can play wmf files with my player withour any problem...

Does anybody can help me?