You may want to play around with alsamixer. Some audio cards have really low volume on some of the mixers like PCM out of the box on Linux.
For a lot of people, louder audio subjectively means better sounding audio. (Loudness wars for audio mastering for example.)
Check if you have qt5-imageformats package installed. It is an optional dependency for kio-extras package that is not installed by default on Arch distros.
I am pretty sure webp works with the wallpaper picker on plasma5 from my experience.