Laya Yoga - references in Upanishads.
I've heard that the methods of creation of mantras (beeja mantras specifically) follow from laya yoga where they use petals of certain chakras (or nadis) OR hear the internal sounds from the body in order to form syllable which is used to replicate a particular form of samadhi later on.
My question is far from Tantra, but particularly from Upanishads only - of references where beeja mantras like - om are heard from inside (internal sounds). One good reference is - Nada Bindu Upanishad which covers internal sounds extensively, but not om sound. I wish to find a Upanishad that covers this particular case.
Other Upanishads such as Yoga Sikha Upanishad do cover laya yoga, but it isn't very intensive or informative either.
Edit 1: I looked upon panchabindu upanishad, I don't find anything very meaningful regarding laya yoga here.