I think Obito changed his voice over all the time in order to hide that he is NOT Madara ot at least to hide that he is Obito.
(He was even telling the Kage and Kisame to be Madara, didnt he? )
I can imagine when the anime comes to the moment the mask is dropping the voice actor of Tobi will change again.
(Maybe they go back to the voice actor of the goofy Tobi because at this time the voice seemed much younger.
However we do not know yet the identity of goofy Tobi so this is really just a guess.)
(He was even telling the Kage and Kisame to be Madara, didnt he? )
I can imagine when the anime comes to the moment the mask is dropping the voice actor of Tobi will change again.
(Maybe they go back to the voice actor of the goofy Tobi because at this time the voice seemed much younger.
However we do not know yet the identity of goofy Tobi so this is really just a guess.)