Meditating on sound, am I right and need sutra reference?

I have done S.N.Goenkas Vipassana and Anapana sati meditation. In both the cases my mind remains active and I have lot of thoughts while meditating. I learnt from Eckhart Tolle that the sensory perception can be portal to silent present moment awareness. In other words being aware of sensory perceptions can lead to meditative states of no-thought or no-mind. When I remain aware of sound, I am in the present moment as sound can only be heard in present moment. So, currently I am meditating on sounds that I hear. I am doing lot of progress as i am having less and less thoughts. Also, after some time i can hear a humming sound which is uncreated which I learnt on youtube is called Anahat Naad, but there is no such reference in Buddhist scriptures on Anahat Naad or any formal sound meditation instructions. I have two questions. Firstly, am I doing right meditation by being aware of the sounds. And secondly, is there any scriptural reference of meditating on sound. thanks.