Install DOOM on Raspberry Pi

Install chocolate-doom:
Chocolate-doom is a shareware port for running DOOM game files (.wad files).
$ sudo pacman -Sy chocolate-doom

Get shareware wad file for playing DOOM demo (you can also use your full game .wad files if you own the game):
$ cd /usr/share/doom
$ sudo wget http://www.jbserver.com/downloads/games/doom/misc/shareware/doom1.wad.zip

Unzip the file, but first install the zip/unzip utilities:
$ sudo pacman -S zip unzip
$ sudo unzip doom1.wad.zip

Run DOOM!!!!
$ chocolate-doom -iwad /usr/share/doom/DOOM1.WAD -window

Forget the -window if you want full screen but it will run slower. The smaller the window, the less math the GPU has to do.  Speaking of which, you can dedicate more memory to the GPU so it runs smoother by following the troubleshooting note after installing Midori. This allows for Midori and other graphics intensive applications like Doom run better by allocating a larger chunk of RAM for the GPU to use.

The End Result, the Fruits of Labor

DOOM Troubleshooting notes:

Sound may not work due to alpha drivers.
Solution:
Logout of Openbox, before typing startx:
$ sudo modprobe snd_bcm2835
Test playing sound file:
$ sudo aplay /usr/share/sounds/alsa/Front_Center.wav
If this works, start Openbox with startx and try running DOOM again.

If you still hear nothing, try this:
$ sudo amixer cset numid=3 X
... where X is 0, 1 or 2: 0=auto, 1=headphones, 2=hdmi
The auto setting defaults to hdmi if hdmi supports audio, and otherwise tries to use the analog/headphone jack.
Restart Openbox and Rerun DOOM.