The sound is the sign of the opening of the consciousness and of the working of the inner Force. Such subtle sounds are very frequently heard by those who practise yoga.
When the inner senses open, or any of them, one sees or hears things belonging to the other planes automatically. What one sees or hears of the other planes depends on the development of the inner sense. It depends on what you hear whether these are the symbol sounds only which have a connection with the sadhana or simply other plane sounds of an ordinary character.