BBC is giving up Real Streams. There will be flash RTMP streams and Windows Media streams only.

.flv container is used for AAC audio files. There are many RTMP Flash stream recorders that you can use. After recording you can extract AAC audio from .FLV audio only files using freeware FLV Extract.

Related thread:
www.bbc.co.uk: How to record BBC Radio 1 stream from BBC iPlayer(RTMP .flv, RTSP .ra)