Mac doesn't play sound louder, no bass as well

I was playing music on my Mac recently and all of a sudden the sound automatically changed. It's not playing louder anymore even if i increase the volume to its peak. Also, it doesn't play bass again. it only plays treble. I first thought it was the media player that has encountered a problem but when i tried playing music with other players including itunes, it was still playing in the abnormal way.