If you really think it's AAC, then you might be able to get the AAC codec for TCPMP.
"RTSP" stands for Real Time Streaming Protocol, which suggests that TCPMP does not handle the protocol "wrapper" around the audio in question.
Although the Pocket PC Media Chart does not specifically mention RTSP, you may want to peruse it for media players capable of streaming.
www.feederreader.com/mediachart.html
Greg Smith
Author, FeederReader - Windows Mobile news, blogs, audio, video, podcasts downloaded direct to your Smartphone and Pocket PC
www.FeederReader.com - Download on the Road