*** Warning - obsessive nerd content ***!!!
I think that I may be making progress...
The phone has 3 mics:
- The mic at the bottom of the phone is used for making phone calls.
- A mic at the top of the phone is used for noise cancellation. (The mics at the top and bottom both pick up roughly equal amounts of background noise, but the one at the bottom picks up a lot more of the speaker's voice. Subtract one signal from the other and you are left mainly with the voice signal.)
- A pinprick hole next to the camera lenses on the back of the phone has a third mic behind it for use when shooting videos.
I played a YouTube music video on the original phone and recorded that using the camera on the new phone. That worked fine. A very clear recording of the sound (and picture).
I then tried recording sound only using the sound recorder app. That uses the bottom mic. If I put the mic on the new phone up against the speaker of the old phone I could make a reasonable recording of the music, only it was much quieter than you would expect.
Detective hat on...
Facing the screen of the new phone - there is a slot for the mic on the left side of the bottom edge of the phone, and an identical slot near the right side which has one of the phones 2 speakers behind it. I shone a torch on the 2 slots and used a magnifying glass to get a good look. Both slots have a very fine wire mesh covering them inside. I assume that is intended to protect the mic and speaker, maybe even improve the waterproofing? Anyway, the mesh covering the speaker is damaged. The one in front of the mic isn't.
My guess at what has happened? I think the phone was working fine until some crud got onto the mesh in front of the mic. Maybe the speaker mesh too. I think the crud muffled the mic, and maybe the speaker. I reckon the original owner (OO) did what I did and Googled for an answer. Various people suggested scraping crud out with a pin. I think that OO started with the speaker grille, intending to scrape both but (s)he was a bit ham-fisted and tore the fine mesh. They then decided not to mess any further with the phone and to sell it before any more damage was done. Probably taking the opportunity to upgrade to a new phone...
The speaker isn't damaged and you can't see the damaged mesh unless you shine a bright light into the slot and have very good eyesight, or use a magnifying glass like me. I think the mic mesh is still clogged. I very carefully scraped it and have managed to achieve about a 25% improvement in mic output. Further cleaning might well yield further improvements. If I don't manage to restore the mic function enough that way, I will try the nuclear option - deliberately cutting a small hole in the mesh to let more sound through to the mic. (Or destroy it completely!

)