Avidemux does it right now and they play on my PS3. You get a warning message in Avidemux that says "may not be supported", but the file is valid and seems to play correctly through my system. I don't have my PC hooked up to 5.1, so I couldn't tell you if VLC plays the sound correctly, but it does play the file.
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I thought it originated with this http://knowyourmeme.com/memes/good-luck-im-behind-7-proxies
Avidemux does it right now and they play on my PS3. You get a warning message in Avidemux that says "may not be supported", but the file is valid and seems to play correctly through my system. I don't have my PC hooked up to 5.1, so I couldn't tell you if VLC plays the sound correctly, but it does play the file.
AC-3 audio can't be put into an MP4 container
AC3 was added to the MP4 spec last year