Using testing branch and only allowing only PulseAudio to update is not a viable option for a rolling release distribution
It is possible to only install pulseaudio without having to update the whole system
by installing pulseaudio-git from AUR , that you attempted originally
Another user has confirmed this works if packages are removed and reinstalled
HP Envy - Sound does not work in Manjaro 20.1 PLASMA - #5 by denizx27
I had first to remove libao, fluidsynth, lib32-fluidsynth and manjaro-pulse - after that I could install
pulseaudio-git - having installed pulseaudio-git I added libao, fluidsynth, lib32-fluidsynth and manjaro-pulse