Install alsa-utils if its not already installed:
pacman -S alsa-utils
Then run alsamixer and use the arrow keys to find the channels called 'Left Mixer' and 'Left Mixer Right'. Highlight both one at a time and push the 'm' key to unmute each channel. You'll notice the two M's in each channel turn to green zeros to indicate they're unmuted. You can now push ESC (or Cancel on the Z) to quit alsamixer and run:
alsactl store 0
To save your alsamixer (audio) settings so they will be restored on the next boot.