Below is a translation of a small extract from Bhai Sahib's writing that sheds light on Naam Abhyaas and 4 kinds of baanis:
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The rasna (tongue) of Punj Pyare with Naam Kamaayee (one's who have earnings of Naam) is the rasna of Guru Sahib. Hearing the Naam Gurmantra from the holy and Amrit-drenched tongue of Punj Pyaare with Naam Kamaayee, under the leadership of Siri Guru Granth Sahib jee, causes wonderous, spiritually-attracting Paaras Kala (condition) and immediately after hearing the Naam with ears, the current of Naam Abhyaas runs in the seeker and this current then never stops flowing in the seeker. In every cell of the seeker spiritual Naam convulsions take place. All organs including the mouth, the tongue, the lips and the throat indulge in Naam Abhyaas. This is the greatness of hearing Naam that the seeker indulges in Naam abhyaas without effort. All by itself, effortless Jaap of Naam starts in the seeker. While Japping (reciting, chanting) Naam, the sound or dhuni of Naam too is heard simultaneously. Hearing of Naam and chanting of Naam both become integral part of the seeker for eternal time and every moment the seeker is indulged in Naam.
This hearing and accepting of Naam continues in all works of the seeker. No moment goes by in which the seeker is not reciting Naam and not hearing it. From the most micro dhuni to most moderate dhuni, and from most Para and most Pasanti baani, Swaas abhyaasi Naam just continues. Whatever kind of Naam Abhyaas(i.e. Baikhree, Madhma, Pasanti or Para), that's the kind of dhuni one hears. The main thing is that this dhuni must be heard. To listen to the Naam Jaap roopi Shabad (baani) through your ears, no matter how micro (sooksham tau sooksham dhuni i.e. from Baikhree to the most sooksham i.e. Para) the dhuni, this is what is referred to as Shabad Surat Naam Abhyaas in Gurmat. The Naam dhuni originating from Naam Abhyaas and containing Paaras Kala definitely and surely attracts the surthee of the seeker and as the seeker becomes one with the dhuni, in the same proportion the seeker's surthee gets coloured in the high divine colours. The abhyaasi seeker himself becomes the one uttering Naam and simultaneously becomes the one hearing it.
The fortunate ones, who right from the time they meet Punj Pyaare, have their surthee one with Shabad Dhuni, they have to make no effort to bring their surthee in Naam dhuni because the mystical attracting power of Naam automatically attracts the surthee into the dhuni of Naam. But ones whom Naam has been done Drir at Amrit Sinchaar and have full faith on Naam, but have not yet received Naam Rass (divine taste of Naam) they have to beat or Khadkaa the khanda of Naam with full vigour with Rasna or with Swaas Swaas and with full effort get the surthee to hear the dhuni (sound) of Naam.
It is of utmost importance to get the surthee to hear the dhuni of Naam. Ones who are misled by some misled persons and by persons not knowing about Naam Abhyaas, start getting their surthee tuned to imaginary pictures of the Gurus, they stay devoid from the Gurmat path of Shabad Surat. By placing the imaginary picture in front of the eyes, the condition of having full surthee on Naam Dhuni is never fulfilled and this wrong doing leads them astray from the path of Gurmat. Except for Shabad Surat technique related to Gurmat, all other meditations and contemplating is Buth-Prasthee (idol worshipping). Trying to make an imaginary picture of unreachable and unfathomable Parmeshar is a futile act. Neither this way one gets dhyaan nor one's surthee gets focused. The whole effort goes waste and rather indulging in this act is doing mockery of Vaheguru. To discover the spiritual secrets only Gurmat technique of Shabad Surat can help.
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Naam abhyaas and the jugti of Naam abhyaas does not change with these 4 kinds of baanis. The verticle movement of shabad within the body still stays the same. Bhai Sahib in his book has written that no matter which kind of Dhuni one is using to japp Naam, the surthee should stay in the dhuni of Naam. This is the game of Shabad and Surat. Please refer to Bhai Sahib's book Gurmat Naam Abhyaas Kamaayee for more details.
How would one describe these baanis in words? According to Gurmat literature, the baikhree baani is the audible sound that we utter using tongue. Madhma has more to do with swaas swaas, as per Bhai Mani Singh jee's Bhagat maala but swaas swaas abhyaas can also be done with Baikhree baani. Pasanti is the dhuni that is heard from hirda and Para is heard in the Naabh Kamal. Depending on the seeker, the dhuni suitable is applied. The safest bet is Baikhree baani i.e. that which is audible. Whatever is suitable for us will itself manifest in front of us. What we are to do is continue to engage in Naam Abhyaas the Gurmat way.
When Baba Buddha jee came to Siri Guru Nanak Dev jee and asked for Naam, Satguru jee judging their avastha gave them Naam and jugti but told them to employ Para Pasanti to japp Naam. Baba jee was not an ordinary being like us. They must have had massive bhagti from previous lives. This is based on Bhai Mani Singh jee's Bhagat Mala. Guru Sahib explained the 4 baanis to Baba jee and then instructed them to employ Para baani to japp Naam. We mortal beings cannot fathom what all this means. We should do Naam abhyaas as our Guru Sahib through Punj Pyaare have instructed us to do. All things will fall in place.
Bhai Sahib jee in his writings have alluded to these different baanis. Bhai Sahib writes about many stages of Naam. You can read Bhai Sahib's books for details.
As for how to reach Madhma or Para Pasanti, it happens all by itself. The aim of the true seeker of Vaheguru is to attain Vaheguru and the Dhuni of Naam is the Guru of Surat Chela. The Surat Chela should just follow the dhuni and it guides one to Vaheguru. All Madhma or Para Pasanti stuff happens as time arrives. Such is the Sehaj marag of house of Siri Guru Nanak Dev jee Maharaj. We should just allow the chela Surthi to follow dhuni of Naam and keep bhai-bhau in mind. Rest falls in place.
Daas,
Kulbir Singh
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥