People who are using M2TS files with Adobe Premiere Pro(especially CS6, CC7) often have the similar problems Premiere Pro missing audio track for M2TS files.
There are 3 ways that might help you resolve your work- Premiere Pro, M2TS files, metadata and no audio:
1. Install an earlier version of premiere pro like 4 – 5 depending what version fits what you do……(DO NOT UPDATE!!!)
2. As far as I know import the ENTIRE CARD, very important, all the files from your card to your PC/Mac, the metadata should be located somewhere in a folder within the card, it’s not on the .mts file.
3. If you only get the single .M2TS video, you can choose to change M2TS clips into another format for Adobe Premiere Pro and relink against the version. It will get the audio back into Premiere. Might be a bit time consuming but it works.