I had a similar problem - my loudspeaker started sizzling terribly after a few hours of use. The same happened with the ear speaker. I bought new ones from AliExprees and got them replaced in a local service center. They worked again for a few hours then started sizzling terribly again.
This phone is a very good idea but is very crappy built - a lot of factory faults - you can check the forums not only here but the XDA developers as well for confirmation. I trashed it and got back to my old BV6000s which is behaving very well for more than 20 months.
I am also facing the same problem. I bought this device from GearBest and within a fortnight, volume fluctuates so erratically, that it feels so embarassing while with friends or relatives, and even being alone, this causes really nuance.