Do you have Soundbooth, or Audition? If so, just rip the Audio for the original MPEG, and do a Save_As PCM/WAV 48KHz 16-bit PCM/WAV and Import that. Drag to the Timeline, with the CTI at the beginning of the MPEG Clip and Snap ON (S key), and you should be fine.
If you do not have either of those programs, or similar, the free audio-editor, Audacity, will do the same for you.
I successully converted it to a wav file in Audacity (I tried it in Soundbooth, but the audio wouldn't play there either).
However, I can't just put it into the timeline like you suggested. My movie is already complete. The clip has already been sliced and diced into subclips and placed all throughout the movie.
Can you think of a way to get around this mess?
So that "long Clip" has become many smaller Instances of that Clip on the Timeline - correct?
In that case, the only workflow that I can think of would be to manually cut up that file too, to match. I was thinking that you still had that one long Clip intact, so you could just replace the missing Audio stream, then lock that WAV to the Video, and then edit away. Sorry that I missed where you were in the Project.
I had a similar problem with CS3 and m2V files. I believe that I just added the ad2ac3dec.dll to the Premier folder copying it from the Encore folder per www.slashcam.com/EN/info/Premiere-CS3--No-audio-on-some-imports-MPEG-258091.html .
Hope this helps!
For PrPro, up through CS3, adding the appropriate AC3 .dll from that version of Encore, did a good job of allowing AC3 Import into PrPro. Not quite sure why Adobe did not add that .dll to PrPro, but it was an easy Copy over.
With CS4, things did not work, and this was through CS4.1. No AC3 Import. Then with CS4.2, support was added, and I assume that it remains in CS5, but others can verify. As so many AVCHD cameras now shoot in DD 5.1 SS with the AC3 CODEC, it would seem logical to have full support for that CODEC in CS5 too.
Note: even with DD 5.1 SS source files, one needs the Minnetonka SurCode DD 5.1 SS Encoder plug-in (optional) to Export to DD 5.1 SS. Otherwise one gets a 2-channel mix-dwon.
Good luck, and thanks for reminding us of the AC3 .dll Copy method. For pre-CS4 versions, that is still the "fix."