When changing the volumn either

dis connect all the external wires connected to the unit.also remove the main power cord from power outlet for a minute then wait for two minutes and check out by connecting every thing as it was.also if still the same problem then it Can be the fault in remote as when you press the volume up or down button the mute option is getting applied /just check out with any other spare remote and confirm the fault.
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This will help. Thanks please keep updated.please do rate the solution positively .thank you for using fixya.