I found this writing interesting, from Yogiraj Shamacharan Development Trust: Hearing anahata sound (10 different sound in sequence) happens at early stage of kriyayoga practice when Prana vibration is reduced to some extent (starts at less than 100k, more than 10k range). With more practice kriyayogi hears Omkara sound. When yogi 's Prana vibration goes below anahata chakra i.e. Prana vibration 100..yogi is engrossed in Omkara sound within. So internal sound is heard well before settlement in kutastha. When settled in Kutastha no sound exist, no light exist, no duality exist...thus beyond all attributes like sound light good bad etc. Prana vibration is zero then.
Hi Willpower the moment I read this I realized it was from Ashoke Chatterjee. He has a distinctive way of presenting the material don't you think?
Here is a book to read if you would like to go further in your studies that is very good. This book describes what some of the experiences in Kriya practice are that can be expected as well as educating the practitioner as to why they occur and their significance.
Yoni Tantra Commentary on Selected Verses In Light of Kriya Yoga