It is usually the first audio packet sent through RTMP audio channel, but due to some unknown reason, it is lost. That's why Flash Player cannot play it: it failed to recognize the information required to decode it.
It is quite hard to reproduce the problem. wowza is set up on my localhost, so I don't think it is network-latency-related.
Is it possible for wowza to provide a valid AAC header when it does not exist?
I kind of understand the issue. What encoder are you using? In general if using an RTSP/RTP encoder then we extract this information from the SDP data that is exchanged. I have not heard of others with this issue. Not sure why you are seeing it. I would need to reproduce it to have a chance of understanding it.