If you haven't seen the new landing page for this site, check it out. I'm quite happy with it.
The initial idea was just to choose something quiet, subtle and a little atmospheric. I'm no musician and didn't want to learn how to make the audio clip yet. So I spent ages listening to royalty-free music snippets and finding something fitting. After a half-hour I had a better idea, remembering this: SYSTEM Sounds. It's a cool project where music is made from astronomy by translating certain properties of starts into sounds. One of the co-founders of the project did a TED talk explaining it: What does the universe sound like? A music tour.
So instead of finding a random, edgy, ambient sound, I remembered that I'd saved an image of Westerlund 2 after it came up as the result for a seperate little project released by NASA a few years ago: What Did Hubble See on Your Birthday?
Here's the result I got, Westerlund 2:
Luckily, the SYSTEM-sounds project had a sonification of the Westerlund 2 cluster. I only made some minor modifications to it (trimming silence, adding cleaner fade in/out). It might not give of the initial vibe I was going for but I think this is much cooler.
Here's the video and some details of the sonification, from Chandra: