Premiere Pro doesn't play AC3 audio stream from .MTS file
It sounds like there is a problem with your Dolby Decoder for the AC3 audio stream.  If you are on a trial or pirated copy then you may not have been able to activate the AC3 decoder that normally comes with Premiere. (Adobe has to pay Dolby for each copy used.)  If you are on a legit copy, something may have become corrupt with the Dolby decoder.  Try doing a deactivation of Premiere (Help...Deactivate) on your system and reactivate and see if it manages to activate the Dolby Decoder.