That bug has been opened on https://launchpad.net/distros/ubuntu/+source/nautilus/+bug/34456

"I have an ac3 audio file which is detected correctly by the file command, but nautilus doesn't recognise it:

$ file bar
bar: ATSC A/52 aka AC-3 aka Dolby Digital stream, 48 kHz,, complete main (CM) 3 front/2 rear, LFE on,, 448 kbit/s reserved Dolby Surround mode

while in nautilus, I am only offered to open it with an audio player if the last four characters of the filename are ".", "a", "c", and "3", in that order.

I haven't included the file as I do not have an appropriate redistribution license.
...
> Thanks for your bug. What version of Ubuntu do you use? Without an example it's not easy to do something about that bug. Could you put the .ac3 somewhere online for some time so we can download it and work on the bug if that still happens with the current version?
...
This is from up-to-date dapper. I'm about to attach a file I've made from the Ubuntu MUM presentation. I now suspect this may happen with a wide variety of ac3 audio files.

http://librarian.launchpad.net/1693455/bar
This is a file made from the first 10 seconds or so of the MUM presentation via:

$ mencoder -oac lavc -ovc copy -lavcopts acodec=ac3 -o foo.avi media/video/mom-3110
<Ctrl-C after 10 seconds>
$ mplayer -vo null -dumpaudio -dumpfile bar foo.avi
<mplayer core dumps, but successfully outputs the audio>"