Para, Pasyanti, Madhyama and Vaikhari

The various stages of the manifestation or the development of sound, right from the navel onwards, are known in Sanskrit as Para, Pasyanti, Madhyama and Vaikhari. Para is a soundless seed, as it were, the very possibility of the production of sound. Pasyanti is a little more pronounced. And the more intensified form is Madhyama; and the audible manifestation of it is Vaikhari. Often, these stages are identified, in the cosmical context, with the four metaphysical realities advanced in the Vedanta Philosophy, namely Brahma, Isvara, Hiranyagarbha and Virat

Para
Para is pure Shakti, beyond name and forms. Turiya state or Ardha-matra of aum

Pashyanti
Pashyanti (Bhava/Feeling) - is shakti differentiated into pure thought ...the causal world or the "M" of the Aum. Pashyanti, a state of abstract knowledge, like an idea which has not yet been expressed in words. This is the playground where Shiv acquires the dual form of The Divine Teacher and the Student. Where one part of him questions and the other part dispenses Knowledge. This is the ground where we look for answers to our problems and queries,where the Divine within us plays the role of the Disciple when we have a question to ask, and then the Divine becomes the Teacher and reveals the answer.

Madhyama
Madhyama (before manifestation - like murmuring )- is shakti further differentiated into form and name... the astral worlds and the "U" of Aum

Vaikhari
Vaikhari (Spoken) - is the grossest form of shakti manifesting as word ...the physical world or "A" of Aum.

In more simple terms Vaikhari is the speech, madhyama is the mental translation of form into name, Pashyanti is the thought form and para is pure shakti coming from Kundalini .

Para is Adya shakti, Pasyanti is Maha saraswati, Madhyama is Maha Laxmi and Vaikhari is Mother Kali.
Para to Vaikhari is also mapped with Chakras.

• Para lives in kundalini
  
• Pashyanti lives in heart
  
• Madhyama lives in Agya
  
• Vaikhari lives in Throat chakra
  

Vaikhari is what we speak through our mouth, which had started from Para. Para is subtle and Vaikhari is manifested form of Bhava/sound.