speakers problem

Hello there... i have Macbook 13'' 2011 ... My inteernal speakers work good...but when i listen to music...right side stops and sub in middle of macbook..leaving only left side speaker working that is hi and medium.... right side also works but like its not there(like its turned up to 1 percent, almost silent).... when i tyrn off and leave it for couple minutes everything goes back to normal...but when i listen music again...after couple minutes all again..... I check everything// everything in spekaer preference is good and also in MIDI audio... all is good... i reset PRAM and all the same...any know? tnx