The subtle body is inherently closed for a significant reason. Once it is made open, the subconscious awakens which in turn emerges our karmas. Hatha Yoga is very easy to practice. However, it produces potent and immediate results to the practitioner. The outcome depends on what is ingrained in the subconscious mind of the student.
Once the nada is heard, slowly pierce through it until a softer sound appears. Focus on this new sound. Then go past the new sound and another one shall emerge. The nada is inside the mind only and the sound is to be heard in the mind only. We cannot create these sounds, they are already there. We have to attune our minds for the nada to manifest.