Have you tried pressing F6 to switch to another sound card?
I have seen sometimes the incorrect sound card is in use for whatever reason.
Another consideration would be to install PulseAudio Volume Control:
sudo pacman -S pavucontrol
With pavucontrol you can change output devices (AlsaMixer lets you adjust their volume, but not switch from one to the other). If you have the wrong device selected, that could explain why you are not hearing the sound that is apparently playing somewhere.