It's been done, a thousand times over.
http://forums.ffshrine.org/f7/how-rip-blu-ray-7-1-channels-98669/#post1844862
or
http://forums.ffshrine.org/f92/how-make-complete-score-detailed-guide-84885/#post2045257
But there's no guarantee that you'll get clean results.
It's unpredictable when sfx, dialog or even different source music will bleed into the surrounding channels.
There was another discussion in one of the other topics in the Film, Television and Classical Music Download forums about trying to do cleaner rips.
Any extensive work done to clean the files for its music alone will cause degradation on the quality, as quite often sound effects and dialog will share the same frequencies on varying levels. Destroying that frequency to remove the sfx/dialog will destroy the music.
Even the strongest software can't entirely, nor accurately, predict the missing data.
You'll need to do double the work to restore any lost frequencies/data belonging to just the music.
And then there's the DRC (Dynamic Range Control) to worry about.
Volume restoration isn't as easy as most think.
Volume loss will also produce artefacts when amplified. The volume compression tells the data streams that it doesn't need that data anymore, so it's lost.
Complete restoration will require heavy modifications to the original audio to achieve bearable results.
The bearable results, however, are all personal.
How much you want to edit and suffice with is up to you.
I strongly recommend working with the original Blu-Ray discs to get 1:1 bit-perfect audio.
Downloading a movie in 720p/1080p will give you only the lossy DTS-Core audio, in most cases.
Scene releases like FrAmStOR will often provide the original lossless DTS-HD MA (DTS-HD Master Audio).
If you don't have access to the original Blu-rays, I guess you can work with the lossy DTS cores.
But keep the results to yourself.
Don't go around bragging you used the superior format of Blu-ray only to have ended up using all the lossy parts.
Lossless audio to look out for when working with Blu-Ray:
DTS-HDMA/DTS-HD Master Audio
Dolby TrueHD
(L)PCM
Recommended software to use:
DVDfab (extracts BD movies, bit for bit perfection, can rib by chapter)
MakeMKV (free while in BETA, (extracts BD movies, bit for bit perfection but rips entire movies though)
MKVtoolnix (free tool to look at MKV files and demux files)
eac3to (used to convert MKV files with DTS audio (DTSHDMA) to WAV, multiple WAV files--one for each channel) with bit for bit perfection; recommended to keep audio in a container file, like DTSHDMA inside an MKV file, or remux audio only to an MKA file)
Any audio editing software will do.
Audacity, Adobe Audition, Goldwave.
Import the audio files with exact same audio properties as the imported files and save/export to exact same audio properties. (ie. 24bit/48kHz saved to 24bit/48kHz).
Avoid downsampling music unless you have to.
Do not downsample the sample rate with your audio editing software; Use eac3to downsample if necessary (BEFORE importing into audio editor).
Downsampling the bit depth is only necessary if the original audio contains noise/artefacts/etc. And requires multiple samples to find the right algorithm for the dithering, if dither is needed at all.
Save to WAV format to retain all quality while/after editing.
When complete and satisfied, save to FLAC/APE/etc to retain quality while compressing.
Create a second set saved to your favorite lossy format: MP3, AAC, OGG, etc.
Archive the finished, lossless-compressed files for future use: converting, sharing, etc.
Don't archive the lossy-compressed files. Converting MP3 to MP3 again will create more data loss.
A lot of reading is required to learn what you're getting yourself into.
This sort of "skill" requires patience and competence.
"When ten hours is given you to cut down a tree, spend the first eight sharpening your ax."