If the recording is on a HDR-FOX with custom firmware, you can do the audio conversion on the unit itself using ffmpeg. Open a telnet session and navigate to the folder in question from the command line. For example to navigate to the 'My Video' folder -
Where the input file is 'a.ts' and the output file is 'b.m2ts' -
Code:
ffmpeg -i a.ts -vcodec copy -acodec ac3 b.m2ts
The conversion is not quick; it seems to take roughly as long as the recording length but it runs without being too resource hungry. Note that m2ts files don't show up in the standard user interface or Web-If, but are visible in a file explorer or by FTP. As an alternative you could install ffmpeg on your PC and run the audio conversion there: this will be a lot quicker.