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.
To switch your audio output between HDMI and the headphone jack, use either the raspi-config tool or the amixer command. See audio configuration