Add audio usage instructions

usage/audio.md

@@ -1 +1,21 @@
 # Playing audio on the Raspberry Pi
+
+To play an MP3 file, navigate to the location of the .mp3 file in the terminal using `cd` and then type the following command: 
+
+```
+omxplayer example.mp3
+```
+
+This will play the audio file `example.mp3` through either your monitor's built-in speakers or your headphones - connected via the headphone jack.
+
+If you need an example file you can download one from here using the following command:
+
+```
+wget http://goo.gl/MOXGX3 -O la.mp3 --no-check-certificate
+```
+
+If you cannot hear anything, make sure your headphones or speakers are connected correctly. Be sure to check your audio configuration settings, as described below.
+
+## Audio Configuration
+
+To switch your audio output between HDMI and the headphone jack, use either the `raspi-config` tool or the `amixer` command. See [audio configuration](../configuration/audio-config.md)