@qhn FINALLY it's fixed....the problem is my realtek hd audio upgrade not supported by dolby version...so I download a file on acer website you given to me..THANKS BRO!!!CHEERS!! :D
I update the Dolby Advanced Audio v2..then when I open it ..It says(The current Dolby audio driver version is 7.2.8000.13 and the software application expects driver version 7.2.7000.7.Please install a valid driver and software...