Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General Questions about Steam- and Retailversion

Where is the difference between the Steam Version and the Retail Version of Divinity: The Original Sin:

I have read that even the Retailversion of the game needs Steam to download/update it, but don't needed steam to play this game.

If I buy the game directly on Steam is it a little bit different download or is it exactly the same? And if it is the same must run steam to play the Steamversion?

Is it possible to buy an Retailversion and get the Sourcehunter DLC-Pack from steam?
And if I get the DLC from Steam, which file format does the soundtrack have?