So... all of your music is somewhere on your /sdcard folder, and when you launch Google Music, and scroll over to "songs" at the top, none of it is listed?
The answer to your question is to scroll over to "songs" at the top, hit the three dots, and select "shuffle all" - that will do exactly what you want.
But if the songs aren't showing up in Google Music, there's another problem. The app is supposed to scan your /sdcard folder automatically and locate media. I would try going into Settings > Applications, locating Google Music, and then hitting the "clear data" button. Then reboot your phone.