After a long time I finally managed to isolate the cases when Siri and other voice interaction apps stop working. It seems to happen when the device is connected to a 5 GHZ wi-fi network.
It happened on mine, at least; when I connected my iPhone 6 plus or my mother's 4s to the airport express, there was no way to talk to siri or to dictate text or to leave a voice iMessage, whatsapp voice notes, facebook messenger and so on.
Thanks to a close friend we performed several tests and finally found a temporary work around.
I have set up my airport base to have 2 networks: "capsule classic" which is the 2,4 ghz, and "capsule 5 ghz" which is the other -my base is a time capsule -, where the airport express is connected wirelessly.
When the phone is connected to Capsule classic, airplay and voice interactions work regularly.
When I try to connect to the 5 ghz, problems start.
airplay works, but, don't even dream to start siri! Nothing to be afraid of, it's just that you cannot talk at all, the only way to unblock the situation is to restart the device; deactivating just the wi-fi and reactivating it, does not solve the issue. VoiceOver talks to you, of course, Airplay transmits music to you, but you can't talk to your phone.
I am really sure it depends on network, I already warned both Apple Accessibility and Bug reporter for this issue.
A mistake made me sure of what I am saying.
I just listened to some music on my mother's 4s, then forgot to turn airplay off from there.
As I did not think to have mom's 4s still connected on airplay, I connected my new iPhone plus again, and surprisingly it did not give me any error. I am used that if a similar mistake happens, iOS warns me that this airplay speaker is alraedy in use by another device.
So I remember to disconnect the first one, even if I didn't want.
Surprisingly again, music started playing back from airplay speakers when I selected it from iPhone 6 plus, , I had no reason to think I was making something wrong.
And, when I pressed home for interacting with siri to have some fun from my iPhone 6 plus, here's the bug coming on again!
I took my mother's 4s to ensure that airplay was off, and, found it was on, so after having told silly to myself, I profited for an extra-test, which gave positive results. Siri worked! iMessage too, voice dictation too.
It meant, my mother's phone is occupying the channel right now. Let's turn it off.
iPhone 4s turned off, completely off as I did not need it and my mother wanted it off.
Now, I thought, I will not need to restart my plus. It will take the channel itself. Or, at least, let me turn wi-fi off and on, it will wake up again. Nothing. I was wrong.
Now, I restarted my plus, and, connected to my 2,4 GHZ network, it finally works, it's finally stable. I have music, I have books, I can talk to my phone when needing siri or voice nontes.
Please, if you have similar siri experiences, make some testing if you can change network from 5 ghz to 2,4, and let me know your results and feedbacks.
I have not understood the disadvantages you can have by switching from a 5 ghz to a 2,4. But, as for now I did not find relevant difficulties and I have my stereo and voice interaction working fine, I leave it as it is, hoping that Apple solves the problem, I just warned them by bug reporter and accessibility team, speaking about the problem with details as I made here.