I’d suggest that you do a simple test using your iPhone 7/8/X and an older iPhone (6 or before). Ensure that “Phone Noise Cancelling” is switched off, under the Accesdibility section of settings.
1. Play any sound from a device with a speaker (constant source of sound).
2. Make an iPhone call from the older iPhone to the newer iPhone (or any phone that allows you to hear the quality of the call) and mute the receiving phone so there no sound feedback.
3. Move the iPhone along the front of the speaker playing the sound and to either side to see how much the bottom mic is picking up.
4. Repeat steps 2 and 3 using the newer iPhone to compare the sound quality.
To my surprise, my brand new iPhone 8 Plus only picked up clear sound when the bottom mic was directly in front of the physical speaker and a muffled sound when to either side.
My iPhone 6 Plus, also running the latest IOS V11 picked up the sound from the speaker and from either side and when further away.
Conclusion that was accepted by Apple Support Chat engineer, was that the bottom mic was too condensed and not picking up the sound from the near surrounding and needed to be taken in for a repair.
He could not explain the difference between the phone as Apple has not issued any technical info to confirm any changes to the bottom mic spec.
Interestingly, testing the mic with audio notepad and other VOIP services such as WhatsApp and Viber don’t exhibit the same problem. I haven’t confirmed with them which speakers they are using.
Tony