While it's unclear what caused it at the time of writing, since at some point new installations of postmarketOS on the PinePhone get the wrong Alsa UCM configs installed by default. Both Edge and v25.12 are confirmed to be affected.
This causes several issues such as distorted call audio, particularly from the microphone, speaker mode for calls not working, and speaker audio generally being quiet.
As a workaround, you can manually install device-pine64-pinephone-audio like so:
$ sudo apk add device-pine64-pinephone-audio
If your installation is affected, this should result in several packages being added and several ones being removed. You will have to reboot for changes to take full effect.
Tracking issue: #4405