when ajna opens, astral lights are seen. when crown opens, the sounds of creation or brahma nada are heard in the head. this is from my experience, crown opening is very distinct as only one thing occurs, nada brahma also known as shabda or OMkar or AUM. you may feel ecstasy or feelings of activity in the head, thats only the beginning, when crown bursts open the sounds of flutes, streams, whistles, thunder, drums, roar of the ocean etc... are heard. chanting OM isnt the real OM. the brain becomes a music box or resonance chamber for the divine OM, and this experience is everlasting, every minute of every day divine sounds are heard. so in my opinion, no the crown cannot open before you have passed past the 5 centers of the body, past the 6th center of astral light, and into the highest or 7th center of brahmarandra where you unite with the divine sounds of creation. Everything has come from OM, and everything will be dissolved back into OM.
"He who knows OM knows God"
-Patanjali
I think that is quite a milestone, as inner silence furthers, you will hear the Sound in Silence and attain to the Supreme Ananda or Bliss or ecstasy.
"I am OM, the sacred word of the Vedas, Sound in silence, heroism in men."-Lord Krishna Bhagavad Gita, Chapter 7 verse 8
So all souls should traverse past matter and light and make it to the supreme source of all creation, OM, which is emanating from the Lord. You will hear Shiva's mridanga (double drum) and Krishna's flute resounding in your very skull, this is no metaphor, it is strictly literal.
