The Elder Scrolls V: Skyrim

It's already been said. The only reason Dark Elves in Morrowind had those voice is because of their environment. There is no reason they would speak that way in Oblivion. However, there's a chance that they could have the same voice in Skyrim as in Morrowind, due to its location near Vvardenfell.

Really? See:

From playing Morrowind, I came to understand that Azura punished the Chimer (gold-skinned inhabitants of Morrowind) for the Tribunal's murder of Nerevar (which may or may not be true) and/or for assuming gold-like status when they manipulated Kogrenac's tools on Lorkhan's heart. The punishment was bestowed on all Chimer, making their skin ash-colored, their eyes red as blood, and their voices harsh. From then on, they became the Dunmer.
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http://www.uesp.net/wiki/Lore:The_Battle_of_Red_Mountain

I assume this is the document referenced. There is actually nothing contained in it that directly notes a change in voice, though if all Dunmer were changed and not all Outlander residents of Morrowind were hugely affected by the ash storms in such a manner, then we have to conclude that all Dunmer have raspier voices when compared to the great majority of other elves.

... Besides, it made them more interesting. The rapier voices made for a nice contrast to the Altmer and Bosmer.
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they became n'wahs