They say you can't, but there's always the loopback recording method, set your favourite audio recording up to record, and select the record mixer input to be WAVE or "Stereo Mix" / "What U hear".
This does mean you have to record in realtime while playing.
From what they say, it seems to be their own format ".aa"
It seems that there may be other options.
1. Riverpast - used to convert, threatened, dropped it, apparently back
2. Goldwave - appears you need to open "all files", and the default use of Directshow to handle unknown formats enables it to open.
If that is the case, maybe even "Graphedit" might be able to.