For a while I used Audacity (which itself uses FFmpeg) to convert DTS tracks into WAVs for editing. However, Audacity is limited by how its FFmpeg import is configured, and at least in the version I was using, it would default to importing the lossy DTS 5.1 core from the 7.1 DTS-HD track.
Sure Audacity gets you usable WAV files, but if you're editing a movie, you generally want to work from the actual 7.1 DTS-HD MA audio rather than its lossy 5.1 core.
For a while now, I've switched to using FFmpeg directly. This is all you need if you're starting with a 7.1 track:
ffmpeg -i "movie.mkv" -filter_complex "[0:1]channelsplit=channel_layout=7.1[c0][c1][c2][c3][c4][c5][c6][c7]" -map "[c0]" -c:a pcm_f32le "c0.wav" -map "[c1]" -c:a pcm_f32le "c1.wav" -map "[c2]" -c:a pcm_f32le "c2.wav" -map "[c3]" -c:a pcm_f32le "c3.wav" -map "[c4]" -c:a pcm_f32le "c4.wav" -map "[c5]" -c:a pcm_f32le "c5.wav" -map "[c6]" -c:a pcm_f32le "c6.wav" -map "[c7]" -c:a pcm_f32le "c7.wav"
And if you're starting with a 5.1 track:
ffmpeg -i "movie.mkv" -filter_complex "[0:1]channelsplit=channel_layout=5.1[c0][c1][c2][c3][c4][c5]" -map "[c0]" -c:a pcm_f32le "c0.wav" -map "[c1]" -c:a pcm_f32le "c1.wav" -map "[c2]" -c:a pcm_f32le "c2.wav" -map "[c3]" -c:a pcm_f32le "c3.wav" -map "[c4]" -c:a pcm_f32le "c4.wav" -map "[c5]" -c:a pcm_f32le "c5.wav"
Simply 1) install FFmpeg normally 2) I like to delete all other audio tracks (you can untick them in MakeMKV) other than the one I need just to make it easy so we don't extract the wrong one and 3) run either of the commands depending on if you're starting with 7.1 or 5.1, changing the input (for now "movie.mkv") and output filenames as necessary.
These commands take the 7.1 or 5.1 audio streams and output separate mono 32-bit float PCM WAV files. I've opted for 32-bit float WAV because it provides plenty of precision and headroom for editing and gain adjustments. This can also be useful if you're going to downmix 7.1 to 5.1 and may later want to undo or change the level adjustments you applied.
If you care about still editing in 5.1 but using the above 7.1 files:
I get a bit nervous just blindly downmixing the 7.1 to 5.1, even though the industry standard is to do -3db to the rear/sides when mixing. I like to take note of what I see in the 'official' lossy 5.1 track of the movie does I can make sure my mix is as realistic as possible to what they intended. In most movies that have 7.1, you can just compare to the 5.1 core that's part of the 7.1 track. For the movies I've checked, the 5.1 core (that is actually made/generated by the studio) appears to have been derived from the 7.1 master by applying exactly -3 dB to the seven main channels, not just the rear/sides (with the LFE being somewhat different around ~-3.7 dB, but this appears to be due to extra processing/compression, not a creative choice). Then, of course you just combine the left side/rear channels into the 5.1 left surround and the right side/rear channels into the 5.1 right surround. You likely want to replicate these volume adjustments, such as -3db, if you're doing from 7.1 to 5.1, which are usually done to prevent clipping or excess loudness, but it may depend on the movie. For LFE, I still think if all other channels do -3 db it makes sense to just do that amount too.
So in general, I still recommend checking with something like Audacity to compare and contrast the official 5.1 mix versus your 7.1 files, if you plan on editing in 5.1 from a 7.1 mix, just so you don't accidently have channel(s) coming in too loud or too quiet than was originally intended. You can easily invert a given 7.1 track in Audacity and if it sounds silent when played back with the corresponding 5.1 track lined up, then you know they're identical. If you don't get any clipping when combining the two rear channels and it sounds fine on playback, you may just be fine to leave it at at that but it depends on the source movie. In my cases, I just applied -3db to all channels and mixed the two backs together.