My totally "expert" legal opinion is that if Skyblivion requires a working Oblivion installation (in other words, proof of Oblivion purchase), they should be fine to reuse voice assets. But nobody really wants to go to court with Zenimax for something so stupid, so they just avoid the issue entirely with new voice acting.
Also, why the fuck would anyone want to reuse OB VOs?
Or anything else for that matter.
About the only salvageable part of the base game was the soundtrack. Not great, but ok.
I wouldn't mind someone remaking the game in Skyrim's or even Oblibian's own engine (with extensively modded assets, mechanics and renderer settings) - except in an inclined manner.