Elvis had a multiplicity of voices. I wouldn't like to put my finger on a number. The one thing he always retained -- and the true secret of his appeal (IMO) -- was a devastating intimacy, beyond Sinatra, beyond Karen Carpenter ...... beyond ANYONE. Listen to even a "light" recording like "Vinero Dinero Y Amor". Elvis sounds real. There is no membrane between singer and listener. Elvis never sang over the song, but always into it. Of course, he gave passionate performances and extremely dispassionate ones, but he could never part that intimate aspect from his voice, even when singing the most mundane material with the most mundane of approaches.
I think it's pretty clear that some vocal training occurred on and off throughout the years. But he always had reams of potential. You can hear the "operatic" ability of Elvis' voice even in the 50's; just give "Don't" a spin, with that deep yawning baritone, kissing the theshold of tenorship. But something happened between the 50's and 60's, for sure. On his 50's gospel recording of "I Believe", for example, his voice is as beautiful as ever, but he can't summon up the power to bring a cavernous depth to the high notes; they sort of fold up into juvenile yelping. If he'd recorded "I Believe" in 1966 at the "How Great Thou Art" sessions (a venue very suited to that song), the results would have been very different. So Elvis' voice developed, both naturally and in the "trained" sense. But there was an incredible talent from the start.
During his short tenure at SUN, where the magic began, and, in many ways, remains unsurpassed, Elvis had a bubbly, unkempt, cavorting vocal ability, perfectly brought out by Sam Phillips. But adventure beckoned and he couldn't stay. Initially, he sounded more or less the same way at RCA, but he seemed to quickly get into exploring the lower register more. Then, post army, he came back with a revelatory "bel canto" style, which may be the most formidable of all his voices. More styles followed. Even in 1963, on "Fun In Acapulco" and "Viva Las Vegas", there's a slightly harder edge to his voice. By 1966, the change was undeniable. By 1968, there was no looking back. In 1969, the masculine qualities had fully asserted themselves, and when he took to the stage in Vegas, there was a slight raggedness to his voice, perfect for adding the right level of world weariness (think: "Suspicious Minds"). In 1970, his voice had a more weepy quality to it -- which can be heard literally when Elvis sings, "You'll be weeping", on "Tomorrow Never Comes". This weepy quality foreshadowed the voice of 1971-1973, but in 1970, it was kept interesting with a more husky sound, as well. 1971-1973 are the more nasal years, where Elvis tended to adopt a more piteous tone, yet the emotional power was as strong as ever. 1974-1975 saw him slowly returning to a deeper quality, while retaining the weepiness; the "TODAY" album is the epitome of this. 1976 and 1977 ushered in more changes, albeit subtle ones, as a thickening that can be heard all across the 70's became even more pronounced. Elvis often sounds tired during this period, even on official record, yet he conquered this for the best of his performances in late 1976 and 1977. In fact, stack the Aloha and EIC versions of "My Way" next to each other; Elvis has a much smoother tone on the latter.
We should also not forget that Elvis consciously did different things with his voice at different times. On "Baby Let's Play House", he begins with a mock hiccup style. On "Blue Moon", he wails in high falsetto. On "Jailhouse Rock", he rasps like never before. On "Reconsider Baby", he sneers. On "G.I. Blues", he cups his voice right down, then blasts off into oblivion. On "There's Always Me", he starts soft, yet closes with stentorian strength. On "So High", he follows the bass line. On "Too Much Monkey Business", he sounds impossibly cheeky. On "I'll Hold You In My Heart", he loudly accuses. On "You Don't Have To Say You Love Me", he sighs and snorts. On "Hurt", he bellows with angry volume. Across the wide span of his career, one may also find examples of ebullient laughter, animal noises, yodelling, a convincing Johnny Cash impression, scatting, various "freak out" moments and on. This was a guy that broke all the rules and tried everything. There is no singer more diverse, more bewildering, more frustrating, more entertaining, more eclectic and more esoteric than Elvis Presley.