Support Music players do not play well when display is OFF

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http://androidforums.com – Hello, My Galaxy Y Duos does not play well when my phone display is off. The sound is not stable. Why this? It happens whatever the media player I use. I bought it some some days ago. Firmware is uptodate, I have enough memory on my SD (the phone always unmounted a 32GB 4class SDHC, so I changed it to a 16GB 6class which I had and all is OK). I would not like to root my phone or, I am rather afraid. Could you help me to fix that. Thank you a lot. (General)