It's a known problem to loop mp3's because of the gap they have in the beginning. But since Flash has programatic control, I tried to use the offset to play it. Still, so far, I couldn't get it to loop nicely this way.
I set start time to 23ms, which according to documentation should loop this sound from 50ms till the end, end then again. No matter what value I tried, I couldn't get it to work correctly. I observed the wave form in audio editing software and the sound starts exactly at 23ms, I tried setting this as start and many other values and couldn't stick it together.
UPDATE <- let's hope this works
From Starling forum: http://forum.starling-framework.org/topic/how-to-use-sounds
I'm assuming that you're not starting from the beginning because the MP3 format often adds a little bit of silence at the beginning of the file. I was able to work around this by making the file a WAV instead. Then, I added the WAV to the library of a Flash Pro document and exported it as a class for ActionScript. I believe Flash Pro ultimately makes an MP3 from it, but it adds some magic sauce to make the MP3 work without extra silence at the beginning. Then, if you're using Flash Builder, you can embed the symbol from the SWF with [Embed] metadata. It's kind of complicated, but it's been working great for me.