In the English dub of the episode The Risks of the Reanimation Jutsu, I noticed Sasuke refers to Tobi as Tobi. But Sasuke shouldn't know at this point that he isn't Madara, as far as I know, so it seems odd. And it occurred to me that I can't exactly remember whether Tobi introduced himself as Madara to Sasuke (I think he did, but I'm not sure). Additionally, Sasuke doesn't seem to react much when Kabuto mentions the real Madara. So what does Sasuke know at this point? Who does he think Tobi is?
During the Naruto Shippuden Itachi vs Sasuke fight, Sasuke figured out that during the Uchiha Annihilation, even Itachi couldn't defeat the entire Konoha Police Force completely by himself.
Sasuke was completely aware that someone assisted Itachi in annihilating the Uchiha Clan, and that was revealed to be:
Uchiha Obito, while he was impersonating Madara. Although Itachi did think that Obito was the real Madara.
Sasuke was also aware of the fact that Tobi had a Sharingan, albeit he had no way of confirming that Tobi was the one who helped Itachi annihilate the Uchiha Clan, due to the masks he wore while impersonating Uchiha Madara.
And when Itachi mentioned Madara, Sasuke wanted to kill him, as if he didn't know he died ages ago. I'm pretty sure after Itachi used genjutsu to show Madara, Sasuke knew he was dead.
So Sasuke could've speculated that Tobi was some random person who had the Sharingan and the Mangekyo Sharingan, or a unknown/random person who just happens to go by Madara, an Uchiha that Itachi couldn't kill, or, using his fairly high intelligence, he could assume that Tobi is the one who helped Itachi with the Uchiha Annihilation.
Tobi has also said in the series, during the Fourth Great Ninja War arc:
Madara... Tobi... call me whichever you wish.
And, it is possible that Masashi (and the animation studio) had Sasuke choose a random one of the two names. This was not touched up on during Naruto Shippuden, so my response (and practically all of them, until this is answered by Kishimoto) will be mostly speculative.
(The things stated above may or may not be correct, and there is another possible reason (likely the more correct one). The anime's main language is Japanese, and there is different syllable count on words with the same meaning across languages. For examplem "Nani" is two syllables, while "what" is only one syllable.) It's possible that Studio Perriot had to make him say Tobi instead of Madara to make the voice recording match the mouth flapping/movement.)