[degraafc]

This is just my own experience, but sound works fine for me with just Alsa (no pulseaudio or apulse). Although even the Arch Wiki [1] says differently.

[1]: https://wiki.archlinux.org/index.php/Firefox#Multimedia_play...

[jacquesm]

It is likely that you have some kind of pulse bridge running without being aware of it. For instance Carla will do that without telling you about it.