How peculiar. When did this begin? Have you installed any new software or apps recently? And when you go to settings > sound, does everything look in place? Please try restarting your phone and we hope to hear back with good news.
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I had this problem once on my RAZR maxx HD.
1. Settings->sounds-> tap "volumes"-> increase slider bar volumes for everything listed-> select ok!
2. go to home screen
3. push volume up button and look for volume slider bar graphic to appear on screen.
4. If NO slider bar appears, turn phone off and back on.
5. look to right of time display for sound icon-> change from SOUND OFF to SOUND ON
6. push volume up button and look for volume slider bar graphic to appear on screen