We know that Vedas are Shruti text which are called Aparusheya. Shruti literally means "which is heard". Rishis heard Vedas directly from Ishvar in the state of deep meditation and hence Rishis are called Drashta (seers) of Vedas, not author of Vedas. Apaurusheya means not of human origin.
Vedas constitutes two parts one is Samhita or Mantra and second is Brahmanas. Aranyakas are usually portions of Brahmans and Upanishads are of Aranyakas. Vedas have Shakhas and each of Shakha had its own Samhita and Brahmans as discussed in this post. So, every Aranyakas and Upanishads are associated with particular Brahmanas of particular Shakha of particular Veda. (except Ishavashya Upanishad which is part of Yajurveda Samhita itself).
Now, talking about Shruti-ness or Apaurusheya-tva of Vedas, there are three opinions as follows:
Some believes that only Samhitas are Vedas.Brahmans are not Vedas but they are commentary on Vedas. A popular example of this belief is Swami Dayanand Saraswati of Aryasamaj.
Many (including I) believes that though Samhitas are only core part of Vedas which are the hymns heard by Rishis in deep meditation and hence called Shruti, however Brahmans are also considered as part of Vedas or forming Vedas altogether with Samhitas as I've discussed in this answer
Recently I found that there is also an argument which states that all part of Vedas including Brahmans, Aaranyakas and Upanishads are Apaurusheya and Shruti from this answer:
As a matter of fact, all parts of the Vedas, including Samhitas, Brahmanas, Aranyakas, and Upanishads, are Apaurusheya or authorless
This sounds interesting to know for me and hence I am interested in knowing the basis behind this statement, mentioned in the answer:
This is demonstrated in Adhyaya 1 Pada 1 of the Purva Mimamsa Sutras; see this excerpt from Shabara Swami's Bhashya.
The excerpt linked is whole of 50 pages having commentary on 32 Sutras of Purva Mimansa 1.1. and hence I find it difficult to locate the arguments I am looking for. So, I want to know what are the arguments of Purva Mimansa Sutra that states or concludes that all parts of Vedas including Brahmanas, Aaranyaka and Upanishads are Apaurusheya and Shruti.
Note: If bunch of arguments in the form of "objections and replies" are much lengthy then a brief summery with explanation and citing only major arguments would be useful and appreciated. Also note that I'm not looking for arguments for proving Samhitas Apaurusheya, I'm looking for arguments that proves other parts i.e Brahmanas, Aranyakas and Upanishads are Apaurusheya.