I've been playing concert flute (with a teacher) for a year, and I still cannot always get clear sonorous sound. Instead, it would often hiss.
I speculate that the reason may be in the airflow -- a wrong angle, or maybe too wide a stream -- but I cannot easily catch it. Are there any techniques to learn to hit the right sound?
The discussion in Blow low notes on the transverse flute is a little more specific than I need now.