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OS: Pop!_OS 21.10 x86_64 
Kernel: 5.15.8-76051508-generic

OS: Pop!_OS 20.04 LTS x86_64
Kernel: 5.13.0-7620-generic

I'm trying to downgrade kernel version. I downloaded 5.13.0 version from https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.13/

I downloaded 4 files

linux-headers-xxx-generic_xxx.deb
linux-headers-xxx-all.deb
linux-image-usigned-xxx-generic_xxx.deb
linux-modules-xxx-generic_xxx.deb

And installed them

sudo dpkg -i *.deb

But now I'm stuck.
Pop!_OS doesn't have GRUB and I have no idea how to set the kernel version to be used on boot.

$ neofetch
OS: Pop!_OS 21.10 x86_64 
Kernel: 5.15.8-76051508-generic

$ neofetch
OS: Pop!_OS 20.04 LTS x86_64
Kernel: 5.13.0-7620-generic

I downgraded the kernel version to 5.13.0.
https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.13/

I downloaded 4 files and installed them

$ ls
linux-headers-xxx_all.deb
linux-headers-xxx-generic_xxx.deb
linux-image-usigned-xxx-generic_xxx.deb
linux-modules-xxx-generic_xxx.deb

$ sudo dpkg -i *.deb

Pop!_OS doesn't have GRUB and uses kernelstub
https://github.com/isantop/kernelstub/blob/master/README.md

I changed the kernel version using the following command (where xxx is the desired kernel version)

$ sudo kernelstub -v -k /boot/vmlinuz-xxx-generic -i /boot/initrd.img-xxx-generic

After the reboot, I see that I'm using kernel 5.13.0-051300-generic but that doesn't solve the problem with volume control.

$ uname -r
5.13.0-051300-generic

So my guess is that Pop!_OS changed something between 20.04 LTS and 21.10 that broke proper volume control in my Bluetooth speaker and it's not the kernel's fault.

Edit: I booted live LTS version with an older kernel

And it turned out that there's no issue with that version - system sound control affects my Bluetooth speaker as expected.

I also booted the live version of my current system (to confirm that there's no issue with my installed version) and I reproduced the issue.

So my guess is that kernel 5.15.8-76051508-generic does something with Bluetooth and sound control differently than 5.13.0-7620-generic.

Edit 1:

I booted live LTS version with an older kernel

And it turned out that there's no issue with that version - system sound control affects the volume of my Bluetooth speaker as expected.

I also booted the live version of my current system (to confirm that there's no issue with my installed version) and the issue with volume control was present.

So my guess is that kernel 5.15.8-76051508-generic does something with Bluetooth and sound control differently than 5.13.0-7620-generic.

Edit 2:

I'm trying to downgrade kernel version. I downloaded 5.13.0 version from https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.13/

I downloaded 4 files

linux-headers-xxx-generic_xxx.deb
linux-headers-xxx-all.deb
linux-image-usigned-xxx-generic_xxx.deb
linux-modules-xxx-generic_xxx.deb

And installed them

sudo dpkg -i *.deb

But now I'm stuck.
Pop!_OS doesn't have GRUB and I have no idea how to set the kernel version to be used on boot.