you could buy yourself dvd audio extractor. it will easily extract the audio (truehd, dts hd, lpcm, etc) and convert to flac for you, all lossless
there are also free tools (eac3to) that will extract the audio from an m2ts and convert to flac. i use dvd audio extractor for its MLP support as i have many dvd-audio discs. i dont think eac3to supports dts hd, only the core lossy track for dts.
there are probably tons of other ways, but in the end i think you need to demux the audio stream from the m2ts, which is a video container and so jriver should always treat it as such
my pipeline for this: makemkv to backup the bd audio to a folder structure. dvdae to demux and repackage the audio stream i want into flac. kid3 to add meta data as needed