Random MP3 Selector and Player
I would improve the following things:


move the parsing argument part into a separate function (extract method)
define descriptions and helps in multiline string constants
use os.path.join() to join the path components - would also help to avoid worrying about a trailing slash
order imports alphabetically (PEP8 reference)


The improved code:

#!/usr/bin/python
import argparse
import getpass
import glob
import os
import random
import subprocess as sp


DESCRIPTION = "Open a random MP3 in the player of choice, or the default player"
DIRECTORY_ARGUMENT_HELP = """
The path to the directory where your music is stored. 
If the path has spaces, wrap the entire path in single-quotes ('/home/ubuntu/My Music/' for example). 
If not specified, the current user's Music directory in their /home/ folder is used.
"""
PLAYER_ARGUMENT_HELP = """
The executable name of the media player to open the MP3 with. 
If none specified, uses the system default player.
"""


def parse_args():
    """Parses command-line arguments and returns argparse arguments."""
    user_home_directory = os.path.join('/home', getpass.getuser())
    default_music_directory = os.path.join(user_home_directory, 'Music')

    parser = argparse.ArgumentParser(description=DESCRIPTION, add_help=True)
    parser.add_argument('--dir', '--directory', '--music-dir',
                        dest='dirpath', type=str, default=default_music_directory, required=False,
                        help=DIRECTORY_ARGUMENT_HELP)
    parser.add_argument('player', type=str, help=PLAYER_ARGUMENT_HELP)
    return parser.parse_args()


if __name__ == "__main__":
    args = parse_args()

    # Go to the directory your music is in.
    os.chdir(args.dirpath)
    mp3s = glob.glob('*.mp3')

    # Actually open the MP3 file, and /dev/null to suppress output messages from the process.
    DEV_NULL = open(os.devnull, 'w')
    execpath = [args.player, os.path.join(args.dirpath, random.choice(mp3s))]
    sp.Popen(execpath, stdout=DEV_NULL, stderr=DEV_NULL)


Note that if you would have been on Python 3.x, the DEVNULL could have been imported from the subprocess module:

from subprocess import DEVNULL