You're right that listening is quite a challenge. So we try to first understand what it means and that will help us hear it right. With the verb "sont", we know that the subject is plural. That rules out anything with "c'est" and leaves only "ces".
- "ces de portes" doesn't make sense
- "ces des portes" doesn't make sense
- "ces deux portes" -- aha! That makes sense and forms a correct and natural French sentence.
Yes! so much of my study has been building up a mental list of what is possible to fit into what I have heard, and your comment about sont is exactly the process.
The next step is trying to differentiate between portes and ports as possible answers. Only the male voice gives a hint with the schwa "e" after portes.
And I'm supposed to be able to do this at the speed of conversation.........one day - Si Dieu le veut!
So, how to decide if we have been "schwa'd" or not? On a complex page, full of phonetic symbols, Wikipedia suggests "The main characteristic of French schwa is its "instability": the fact that under certain conditions it has no phonetic realisation."! See https://en.wikipedia.org/wiki/French_phonology