Disclaimer: I'm not responsible for any damage you do to your phone or to yourself or whatever by following the instructions below. It's just a shared experience written in imperative style.
For unknown reasons the lower speaker broke. (CSD-Tool said "right stereo speaker" when testing it)
Decided to replace it by myself. Sorry for the very poor image quality, I used a decaying HTC 1 for taking these photos and never checked its ability to make photos in general.
So, what to do?
First, we need a replacement speaker. I ordered two and tested them with the headphone output of my good old Jolla1. Surprisingly they had a noticeable difference in volume between each other. I took the louder one.
Second, we need some tools: A heat gun, a plectrum, plastic levers, sucker.
We use some heat weakening the adhesive. Be careful with the heat gun, use it on the edges, spare the camera, just give it a little warmth:
Now the rude part.
Sorry again for the lousy picture quality. I hope you can see what it is intended to show. Seems the HTC cam gets worse the longer the app is active.
Try to not touch the inside of the back cover with your skin, because neither the adhesive nor the copper contacts nor the camera glass loves grease too much.
Now we have to remove the plastic frame, covering the board. There are 3 srews:
The L-shaped frame is clasping the bord beneath, and should be removed by carefully unclipping from both sides. Some force is needed.
The lower speaker is located in the metal plated box on the left side of my tool.
Here the frame is gone:
The tiny board with the coaxial cable can be removed by shifting a bit to the right. DO! NOT! TOUCH! the coaxial plug, leave the board attached to the cable instead.
But we need to open the signal connectors, both on the board to the right. Just pull the flex cable up, or use a plastic lever. You see both connectors in the next, slighty better picture:
Why? We need some space to lift the board on the right, because the speakerbox is locked under it.
The next steps are not documented in pictures: fold the whole speakerbox up to get access to the clamps of the metal shield. There are 3 clamps: on both sides and one on the right end. Release the plastic box from the metal shield by lifting the clamps. You will need some force because of the adhesives there, you can see it as black trail in the next pic. Once you have the blastic box seperated, you will find the speaker glued inside. Use the lever to get it out.
The pic shows the state with the separated plastic box, the speaker still in (the speaker is the metal part with the big arrow on the left) :
consider the mounting direction of the spare speaker!
There are sets with spare adhesives purchaseable out there. There's one for the speaker carrier, so if you like to do things perfectly, you should buy one along with the speaker, also one for the back cover.
For myself I used just some (not much!) glue for the speaker itself. Maybe next time will be a bit harder to remove it...
Be careful while reconnecting the plugs on the board. They have microscopic size, but need a little pressure to snap in.
The backplate can be attached without any issues. I used the old adhesives again, but they are available as spare parts. After attaching I applied some heat with the heat gun. Normally I use the phone with a protection cover, so there will never be force to the backplate. If you prefer to use the phone bare, maybe you should invest some more care to the adhesive connection of the back cover.