On Arch Linux all the channels are mute by default. To unmute your sound card you can do it manually in the ncurses-gui-based alsamixer of the alsa-utils package which looks like shown on the following picture.
MM means that the channel is muted wheras OO marks it to be unmuted. The vertical bars visualize the volume which can be turned up and down with the up and down cursor keys. The Master and PCM channels need to get unmuted to hear some sound from your computer.
The same job can be done with amixer with the sound card number, the channel and the volume controls as arguments.
amixer set -c 0 Master 25 unmute amixer set -c 0 PCM 25 unmute
Each sound card is enumerated, the first is 0, the second is 1 and so forth. To find out the appropriate number of a card the declaration can be found out by executing aplay -l.
➜ ~ aplay -l  **** List of PLAYBACK Hardware Devices **** card 0: I82801CAICH3 [Intel 82801CA-ICH3], device 0: Intel ICH [Intel 82801CA-ICH3] Subdevices: 0/1 Subdevice #0: subdevice #0
The settings are stored in /var/lib/alsa/asound.state, to reload them after a reboot, the alsa daemon can be added in your /etc/rc.conf (on distributions not following the bsd style the system configuration file might differ).
... DAEMONS=(... @alsa ...) ...
The @ symbol means that the daemon is started in the background which speeds up the boot time.
Other useful commands can be found here: