After hours of messing about i finally got a pi that would automatically shuffle a playlist at boot so only one button was needed (the on and off wall switch). Mpg123 just wouldn't work for me at all so i added shuffle capability to the built in omxplayer. This was also done with no pi internet connection.
I'll write my steps so no one has to struggle with this (if you don't know how to do a step, google is your friend
1) Flash Raspbian to a microsd/sd card
2)Download Python setup tools (this will allow you to install omxplayershuffle)
3) Download Omxplayershuffle (this will let you shuffle your music easily using a simple command)
4) Gather all your music in an mp3 format
5) Transfer the files from steps 1-4 to a usb
6) Make a music directory on your pi (a folder)
6) Transfer the files from a usb to the pi (make sure your music is in the music directory)
7) Extract and Install Python Setup tools first and then Omxplayershuffle (there are tutorials for extracting archives on a pi online and how to install each program on their respective websites)
8) navigate to the /etc directory of your pi then add the following to the end of your /etc/rc.local script:
cd "your music directory here"
(the command to edit the script is "sudo nano rc.local")
WARNING - it will be hard to modify anything on your pi after step 9 so make sure you have all of your music in place.
9) Reboot your pi
10) Plug in a speaker to your headphone jack and to the wall/usb port on your pi (if it provides enough power), set the volume of your speaker to something comfortable if your build is for someone who won't be able to work the volume switch, otherwise the volume switch on your speaker will be used to control the volume.
Bonus: 11) You could Build a shell around your setup and add an on and off switch that controls the power to the unit that's separate from the one on the wall in order to make this more of a unit.