"But mpv works with pulseaudio, right? What are you talking about?"

Well, that changed since the last release.
The default is now handled by "softvol". The audio (when oa=pulse) is still being send to pulseaudio, but the volume keys and default volume are not.

So that can end up in the situation that:
- start mpv (pulseaudio restores the volume to whatever it was, say 30%)
- softvol always starts at 100% which is 100% of that 30%.
- if you now use the "/" key to decrease volume then it's decreased from the softvol value, not pulseaudio. Same with increase.

That's not what you'd want and most certainly not what you'd expect in this time where everything at the very least works somewhat ok with pulsaudio.
MPV also added now key commands that you can bind to get the old behavior back, but they are not setting it for you. So the user either has to do that, or the distribution has.

I'd like to request Arch to provide default bindings for the default volume keys of mpv.
SHIFT + * is by default used to increase volume
/ is by default used to decrease volume

This can be set in input.conf (system wide would be at /etc/mpv/input.conf)
It should contains the following entries at the very least:

/ add ao-volume -2
SHIFT+* add ao-volume 2

There are probably more volume keys.
I know Arch is all about sticking to upstream and leaving all config up to the user, but this one is quite vital and difficult to discover and correct.

Cheers,
Mark