Call receiver sound low

Hi! My wife has been using this phone for 10 days. First days it was all OK but suddenly sound during calls started to be very low and I cannot ta make it louder. It is hard to hear th e opposite side talking. When listening to music or youtube everything is OK. I have entered enginerring mode but all can I do there is to test sound, cannot change levels. I think it's maybe a software problem. i ma on o Chines stable ROM 7.1.2.0.

Thanks for ideas, it's really a great problem because you cannot talk to other persons normally

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