Comment 6 for bug 585587

I have no idea why this happens. But I found a solution after about 5 minutes.
Go to System > Preferences > Sound
Go to the Applications tab
Keep that open and open a nautilus window with a sound file
Hover your mouse over the sound file, but keep the sound preferences window active.
After a second, you will see a seek bar called Audio Preview. Do not move your mouse.
Use your down arrow button to go down to the seek bar, then use the right arrow button to move it to 100%. That should do it.