The reason is:
Adobe has updated Flash player; and the latest Flash version doesn't play our Buffered audio-only live sources, which is exactly your case.
For now, you need to switch your live source to be Real-Time.
In a couple of weeks, we are going to release version 7.5 of Live and Media servers, which will address this issue;
and also will have AAC audio encoding which is handled better by Flash player.