Ever since I saw a rather interesting video on YouTube of somebody using the noise reduction’s “residue” function to get a profile of a part with no singing and apply that to the song to make an instrumental, I’ve experimented with the noise reduction plugin extensively.
The first thing is: The results will not be usable in any way whatsoever for professional / semi-professional remixes / mashups / what have you. This is merely for curiosity sake, and I personally just do it for fun and nothing more.
For “removing / isolating” an instrument / vocal track, it highly depends on the audio content and the similarities between the parts with the vocals and the parts without. If the song is repetitive and has solo’d instrument portions in the mix then maybe you will have some luck. I actually upmixed a mono performance snippet to a stereo file using noise reduction to get the sources. It’s very computer-y, but it was just an experiment.
Original video (mono):
What I did was I copied the instrumental portions to a new track, made a noise profile of that, and set it to “residue” so I got just the music. Then I opened up another instance of Audacity and did the same procedure, but set it to “reduce” so I got the vocals. I split the music into the 2 channels and added a delay to the right channel. I then made the vocals single-channel mono and added reverb to them (vocal II preset). Oh, and after I mixed it up I generated a new white noise track and used noise reduction to reduce the hissing in the original recording. After that, I mixed the newly created stereo track into an MKV file using MKVToolNix GUI.
Drum tracks can be “isolated” by getting an unrelated drum track (anything with percussive characteristics works, but if possible try to get an isolated drum part in the original mix since it’ll fare better results) and have it make a profile of that and set it to residue. Lower the sensitivity a fair bit (4 or 5. any lower will make it sound hollow).
Bass tracks can be done the same way, but the clarity of the results wildly vary from song to song.
Vocals can sometimes be isolated to the point where it’d be suitable for a non-professional remix by using the Vocal Reduction & Isolation plugin with the preset Isolate Vocals, at strength 50 and the low-cut set to maybe 180-230. Then just reduce the remaining instruments by using noise reduction to capture profiles of the instrumental parts.
In turn, you could make an instrumental track by downmixing this “isolated vocal track” to mono (since it’s dual mono no info is lost) and normalize it to 0 dB, and then just reduce the remaining instruments like I said earlier, and then you can just use the “Invert” effect on it and when the original and the “isolated vocals” play, it’ll cancel out the vocals and leave the drums more intact, all while still being stereo!