I have a number of webcams streaming h.264 from FMLE via a PC. Although these streams do not have any audio (they're just webcams), the only way we've been able to get them to work via Wowza is to include an 18K audio component (MP3) on top of the 250K video stream.
I would now like to push these streams to iPhones, however I have read that the audio codec needs to be AAC. This codec comes by default on a Mac, but not on a PC - it's a MainConcept plugin at $180 per unit.
The problem I have is that with 20 cameras, it's hardly economical to pay $180 per codec per camera (ie $3,600) for an audio codec that is only required so that the stream will work via Wowza.. ie I get absolutely no benefit from the AAC codec!
MP3 audio is supported in iOS devices. In previous versions of iOS (before version 5) it had to be stereo, 44100Hz and 128kbs to work. iOS 5 supports lower bitrates. I would stick with stereo 44100Hz, and 128kbs.
As far as I know, only Smooth streaming to Silverlight has limitation of video only or audio only stream. Video only should work on iOS, Flash, RTSP. And MP3 works on iOS, as I mentioned. Roku is the only Cupertino client that requires AAC that I know of