Here is the process I used to convert some DVD-audio ISO files into proper FLAC files for inclusion in my main collection.  No compromise in channels nor in audio quality required.

These instructions are for Ubuntu but will likely work on most Linux distributions (since ffmpeg and flac are so common).  Mac?  Maybe.  Win10 with bash?  Probably.

The basics are as follows:

• mount the ISO (or extract the relevant files)
• use ffmpeg to extract audio data into wav files
• stitch multiple wav files together using Audacity or ffmpeg
• split that full-album wav into individual songs
• convert those wav songs into FLAC files
• tag and move into the main collection

Often mounting an ISO is a matter of right-clicking and choosing Mount; regardless I leave it up to you to make that happen in whatever manner you choose.  All that matters is that we can access the files through a specific file path. (Alternatively you can extract the relevant files to an accessible file path for the subsequent commands.)

You will need to fun ffmpeg against each audio file from the mounted ISO in order to get a complete collection of wav files to conjoin.  My most recent ISO had two and this command is for the second extraction:

##
ffmpeg -i ~/Desktop/ISO/AUDIO_TS/ATS_01_2.AOB ~/Desktop/RW/02_end.wav 

# run ^^this^^ line for each file containing audio and with a unique file name (in order) 

# you can use the following line to stitch two (or more by modification) wav files into a single long file 

ffmpeg -i ./01_start.wav -i ./02_end.wav -filter_complex '[0:0][1:0]concat=2:v=0:a=1[out]' -map '[out]' full.wav 

# you can see above you'd need to add more -i (input) files and you'd want to alter the bit before and the bit after concat to correspond with the correct number of files 
##

Either use the above command to stitch all files together or open all your files in Audacity (or whatever you’d like) and paste each track against the end of the previous track until you have one project containing the entire audio content.  Export that as a single wav file.

Audacity will default to exporting as a two-channel 16×44 wav file.   You can change the bitrate in the lower-left of the project window.  For multi-channel exports, you’ll need to navigate to Edit → Preferences → Import/Export → When exporting tracks to an audio file → Use custom mix (radio button).  Now you are set to wave the full wave file.

Next you get to struggle with dividing the full file into individual songs.  This can be made all the more challenging if your album is progressive rock.  Who needs gaps between so-called songs?  Anyway, I can not offer much advice though there is this handy page from Audacity that could help.

You can do it!

Converting into FLAC is really easy.  It’s probably the easiest step in the whole process.  (Splitting into individual songs is probably the least easy if you’re keeping track.)

##
# cd into the folder where your newly created wav files are located and run flac itself 

flac --best --channel-map=none -V *.wav 

# you only need --channel-map=none if your full-length file has more than two tracks  
##

Once that has finished, you can tag your music.  I use EasyTag because it’s easy.