Apologies, was a public holiday in UK yesterday and I was at my grandfather’s… Now! Awful lot written about the Cobain roar, his particular skill for being able to hold a single note while screaming, to be able to twist a scream up, down, wherever he wished it — that’s a genuine technical ability on display, not just an unpractised gut talent. And that’s what has made the various vocal-only/acapella versions of Nirvana’s songs so interested. None have been officially released (as far as I know) and I can’t imagine they ever will be although voice only versions of hip hop albums are not an uncommon phenomenon given the desire of remix artists to get working with individual voices. Given the rather unlovely nature of a hip hop vocal, those releases are usually best for the appreciation of lyrics, for the dexterity with which an individual plays with syllables and their control over speed and breathing.
In the case of Kurt Cobain’s voice, isolated from the instruments, there’s a wide range of material to choose from stretching from early efforts — the very gruff, paint-stripping growl on Negative Creep or thick tone of Blew — to the frailty that has crept in on a song like Very Ape or All Apologies. Of course I’m sure that what we’re hearing is not necessarily the development of a voice but deliberate decisions regarding what to emphasise or discard — we’re hearing control, an expanding and experienced talent.
One major contrast is being able to hear so clearly the work done to the vocals on Nevermind. The slight echo on a track like Drain You softens the edges; check the almost syrupy effect added to In Bloom alongside the more extreme doubling of Kurt’s voice on the chorus; the demo like quality of Something in the Way is a welcome release with the visibility of each vocal tic and slur now starkly present. There are plenty of sources and certainly their availability is well-known across the fan community:
http://www.youtube.com/watch?v=pxiU5AwVqhU&playnext=1&list=PL3D72AAD6A809BC28
The additional touches aren’t as visible on the other albums, they’re there, of course, but the nakedness of the voice is plainer. This is my particular favourite. The gulf between the downbeat verses, the ability to let the voice break over a note — I’m very sure it’s deliberate, he did it so perfectly during Where Did You Sleep Last Night from MTV Unplugged — the building snarl of the bridge then the sustained chorus…And yes, there’s plenty of doubling going on at the end there but still…What a song and what a voice.
MV (*cough*) is also available in this form showing him stretching his voice from the initial croak to the dredged up choruses…Another late era experiment with the voice.
Nirvana Legacy 
I just want to say that finding this post has been the most gratifying experience I’ve had in a while. Been thinking about Cobain a lot after watching Montage of Heck recently, and this time extensively about his VOICE. You described what I’ve been unable to articulate about it SO PERFECTLY!! Thank you! And now I’m inspired to share my own thoughts:
I think my favourite album in terms of what he’s able to achieve with his voice is Incestide, and that may be because I like to think that at that time he was still having fun with exploring what he can do with his voice after already proving that he can primal scream better than most on Bleach. Big Long Now has always been a favourite – with that nearly shrill wail after those dreamy/drugged-out verses, but without the “gruff, paint-stripping growl” as you mentioned, which seems to be his default. Many other songs like Mexican Seafood et al. you can really get a sense that he was playing around with pitch, tempo, and pausing to match the sentiment of the lyrics and the melody itself PERFECTLY. Like that song kind of makes me feel sick, which was obviously intended. I think Sliver would be impossible to be covered by anyone else; try to find someone that can make a song about being a bratty kid sound all at once sarcastic and sincere, instilling child-like lyrics with the enraged intensity that he’s used to in almost every other song, and choosing the vulnerable phrase “Woke up in my mothers arms” to wail out the longest and hardest.
I think for In Utero, he was changing his overall style from hardcore grunge to a more acoustic sad-core a’la the Unplugged performance, and songs All Apologies and Heart-Shaped Box, and as we later saw Do Re Mi. He was about to collaborate with Micheal Stipe for chrissakes. He seemed to realize the limitations of his genre and his voice really seemed to adapt readily. That voice alone is what makes his simple demos and covers so coveted still, he can make a simple song haunt you for weeks.
I actually dislike many of his later performances, I feel like I can tell that he was lacking the passion to be adventurous with his vocal performances. Compare the recorded and later live versions of Serve the Servants and Heart-Shaped Box. Live, they were delivered warbly and husky which kind of reminds me of those shallow parody bands like Puddle of Mudd and Nickleback.
Ending this with what I think is the best version of School: the way he extends his voice the second time he repeats “no recess” during the chorus… I still remember watching that for the first time as a teenager and promptly having to chain smoke afterwards because it gave me such intense goosebumps.