* Stereo mixes of officially released material: The most you can do with that in order to uncover 'hidden gems' is playing with audio manipulation (e.g., muting channels, inverting phase, cancelling frequencies, etc). For instance: http://youtu.be/WE9qVsKCrww?t=4m17s
* Officially released instrumentals (or nearly instrumentals): That includes GKH, The Eye and some bonus tracks here and there (e.g., the YMBF backing track from the 2011 ANATO). Not having the lead vocal, you can hear more clearly things such as bass, harmonies, etc. Not all Queen tracks are available in these formats, though.
* 5.1 Surround Mixes: A bit of an early 21st century trend, as several albums were being re-issued to be played in home theatres rather than stereo systems. That way, things could be spread more: instead of cramming nine voices in a stereo mix, you could pan some of them in front right, some in front left, some in surround left, some in surround right, and leave the lead in the centre (although it was usually also audible in the surround channels, depending on the mix). By isolating the five of them (sometimes four, when the mix didn't have a centre channel), some nice things were uncovered, such as the two seconds of piano on Princes of the Universe which I personally would've never noticed otherwise. If you mix and match and apply some more audio manipulation, you can get some nice things too, such as Brian's isolated falsetto at the end of '39. Again, not all Queen tracks are available in this format. In fact, only GH, GHII, ANATO and The Game, so far.
* Stems: Thanks to videogames and computer/electronics/technology experts, when some Queen (and other artists') songs were included on Guitar Hero, Rock Band, etc., the record labels and/or producers and/or artists' management supplied the game developers with the multi's, which allowed certain 'stems' to be created (e.g., just guitars, just bass, just piano, just vocals). That's why we can hear those isolated bounces in those cases, and a lot of things have also been 'discovered' thanks to that, such as the double-bass on Death on Two Legs. Myself, I've also loved hearing other bands' and artists' stems and hearing some nice gems, such as the acoustic guitar on Maxwell's Silver Hammer (never noticed it before, and AFAIR most sources miss it), or just the voices on loads of things from Ziggy Stardust to Poker Face. Queen and Queen-related of which stems are so far available are, as far as I know, in chronological order:
- Keep Yourself Alive
- Seven Seas of Rhye
- Killer Queen, Tenement Funster, Now I'm Here, Stone Cold Crazy
- Bohemian Rhapsody, I'm in Love with My Car, Death on Two Legs, You're My Best Friend
- Long Away, Somebody to Love, Tie Your Mother Down
- We Are the Champions, We Will Rock You
- Bicycle Race, Fat Bottomed Girls
- Crazy Little Thing Called Love, Play the Game, Another One Bites the Dust
- Under Pressure
- Hammer to Fall, I Want to Break Free, Radio Ga Ga
- One Vision
- I Want It All
- Show Must Go On
- C-lebrity
* Multi-Tracks: The most we can get in terms of isolation. Not just having bass, but bass as tracked via DI, as tracked via amp and as tracked via external mic; not just backing vocals, but (sometimes) each part separately; not just guitars, but lead guitar on one track, rhythm on another, another rhythm on another, etc. Sometimes, because they only had 16-track (first two albums) and 24-track (SHA up until around The Works, although NOtW and Flash were probably 32-track), sometimes things had to be bounced and the possibility of isolating them any further was gone for ever. That's why we can listen to the Brighton Rock choir, but not each part separately. Same with some guitar harmonies (e.g., KQ near the end). Four Queen songs have leaked this way: Bo Rhap, Killer Queen, Brighton Rock and Get Down Make Love.
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That's it.
So the degree of isolation depends on which group the song belongs to. If you wanna hear just the guitar on Tenement Funster, that can be done. Sleeping on the Sidewalk? No. At least not yet. Same for vocals: wanna hear Radio Ga Ga a cappella? You got it! All God's People? Not available.
