Via JIPS, where it is the link of the week: the Curiae Project collects U.S. cases, including attorneys’ briefs and oral argument, ranked according to the frequency with which they are searched for.
It is a collaboration of the Library of Congress, Supreme Court of the United States, Supreme Court Historical Society and other institutions.
The most commonly cited and most frequently viewed cases have links from the opening page, and there is also a search function.
One of the links is to Oyez, described as The Supreme Court Multimedia, where there appear to be MP3s of some cases.
The site isn’t complete yet, and it calls itself a beta version.