Assuming you want a music file to play from a defined folder of files, this is one way to go (open a terminal to execute the commands):

1. Choose the music player:
   
   
   I am suggesting vlc but other choices like mplayer also work. If you have only ogg files canberra-gtk-play or paplay is fine.
   
   
   Playback check:
   
   cvlc --play-and-exit path/to/Music/song.ext

mplayer path/to/Music/song.ext

paplay path/to/Music/song.ext

canberra-gtk-play -f path/to/Music/song.ext




2. Create a new script:
   
   
   sudo gedit /usr/bin/play_rand_login.sh
   
   
   and paste the following:
   
   
   #!/bin/bash

files=(/home/me/Music/*)

cvlc --play-and-exit "${files[RANDOM % ${#files[@]}]}"
   
   
   Change the music path /home/me/Music to your actual folder.
   
   Replace cvlc with your player choice and close the file.
   
   Now make the file executable:
   
   sudo chmod +x /usr/bin/play_rand_login.sh




3. Open the Startup Applications Preferences, choose the Gnome login sound and edit the command to read:
   
   
   /usr/bin/play_rand_login.sh




4. Logout and login to test if all is well.

Notes:

• It is not guaranteed to have a different file playing at two subsequent logins but if you have many files the chances are high.


• Tested to work at on Ubuntu 12.04 (using Gnome 3).


• The bash random file choice is coming from a stackoverflow question.