Although I'm not that knowledgable about the lore, I understand that there must have been hundreds of people throughout the eras who were able to use the Voice; not even counting the greybeards.
However, how is that possible when you need to "absorb dragon souls" to learn new "shouts"? How reasonable is it to assume that all of the users of Thu'um were professional dragonslayers, and possibly even dragonborn themselves? And how did the greybeards keep the art of Thu'um alive when there were no more dragons around to kill? What is the point of founding a college of the voice when every apprentice has to kill a dragon just to start Thu'um 101?

Is there anything in the lore that makes the whole connection between absorbing dragon souls and Thu'um any more reasonable? Or is it a classic case of rewriting the lore to fit a new game? :shrug:

You don't need dragon souls to learn them. A Dragonborn can just learn them faster and easier than others, and can supposedly do it better than others.

You need the souls to read the word wall. Not to learn the language or use the shouts.

While I can't say for certain how it works until the game comes out, I'd say it has to do with the Dragonborn thing. If you get a Dragon Soul, which Dragonborn can do, then instead of needing to learn the whole language you just look and the right word sort of lodges its way into your brain in the right place to be used.

Talos didn't need to kill any dragons to make a Thu'um omelet. That is, he only listened to the Graybeards chant until it almost killed him, and when he left High Hrothgar, he possessed the power of Thu'um.

According to Arcturian Heresy though, his powers were merely the assistance of Wulfharth the Underking. It's still pretty accepted though, that Talos had the power of Thu'um and was Ysmir, so one could just end the argument by accepting that Wulfharth and Talos are both reflections of the same being (Enantiomorph), and both are Ysmir, so when Wulfharth helps Talos, it's really no different than Talos just using Thu'um.