Hey jobraun,
I currently do exactly the same way you mentioned which is:
- POST SoundCloud's stream URL link to EchoNest
- Retrieve track ID in the response
- And then wait to query the track ID again and retrieve the audio_summary
I have not found any way to asynchronously be notified of the completion of the audio_summary which is a bummer. Also, I believe I saw somewhere that a track analysis stays alive for approximately 24 hours so you could maintain a cache list of all track IDs and SoundCloud id for a day. This way you might be able to save yourself some time on successive queries for the same track on the same day.
But if you know any way to asynchronously be notified for the track analysis' completion, let me know :)
Thanks, Abhishek.