Hitler didn't like to be interviewed, so he avoided it if he could, especially after 1933, when he was in a position to decline them easily.¹ There are a few audios of him talking though, but not in a interview context, they're more like a personal conversation, where he has a completely normal voice.²

Sources:

¹ http://www.jstor.org/stable/4634275, an interview conducted in 1923: http://www.theguardian.com/theguardian/2007/sep/17/greatinterviews1, and a book about two interviews from 1931: http://en.wikipedia.org/wiki/Unmasked:_Two_Confidential_Interviews_with_Hitler_in_1931

² http://www.spiegel.de/international/germany/historian-interview-new-book-explores-personal-charm-of-hitler-a-927155.html

When Hitler arrived at Mannerheim's headquarters in 1942 for the Finnish Field Marshal's 75th birthday (a surprise visit that did not really wow the Finns as intended), the Finnish secret police recorded the Hitler-Mannerheim conversation until discovered by Hitler's security detail and disconnecting. This is one of the few recordings we have of Hitler's normal (rather than speech) voice.

He's rambling and monologuing, as he was known to do when in private.

You can listen to the conversation (in German, of course) with subtexts in English here.