Open your text messages, tap the option key, message settings, scroll down to the bottom and make sure that the notifications box is checked, then select your tone.. if that's done already....
Go into system settings, sound, volumes, and make sure your notifications volume isn't turned down..
Also, my device will sometimes play notifications really quietly... has ever since ics..
DROID RAZR MAXXIMIZED!!!! PREPARE TO BE VANQUISHED!!!