Slashfilm.com has the transcripts of Stern's radio show, but the gist is basically this: He was allegedly asked if he wanted to provide some words for the character of Soundwave, a little radio that transforms into other things. Stern's first reaction was "hey, cool," but this his agent stepped in and said something like "that's way too skimpy a role for someone of your cultural stature!" (I'm guessing.) No word yet on who they hired to voice poor, unloved Soundwave.
The source does ask one compelling question: Why was Stern contacted in May for a movie that's opening in July? Aha! Some commenters at Slashfilm make an interesting point: It's more likely that Stern was asked if it was OK for his voice to be used when Soundwave kicks on. Like a brief snippet of an old Stern broadcast, most likely. Well, either way, it's not happening.