Here's what fixed what I think was the same problem with a recently acquired SM-A037U although I described it as glitchy and crackling; over time it would build up to start causing playout to slow and would even affect loudspeaker output. The solution was to report it to Samsung Tech Support who authorised the phone's return to a service centre which replaced the GH81-21867A SVC JDM-SPEAKER_MODULE_SM-A037U_SVC;SM-A. Before seeing the Service Order Sheet, I was told over the phone that they had replaced the speaker and microphone. That hardly seems intuitive but they are integrated in a module that must contain electronics such as amplifiers, analog-digital converters and quite possibly digital buffers, data demultiplexer, audio encoders/decoders and maybe a clock that needs to synchronise with the main board. And maybe all that has a microprocessor at its core on some version of firmware. Possibly there was bad batch of these modules.
So, if you are experiencing this problem, skip the multiple factory resets and options twiddling that I went through and get on to Samsung. Hopefully, you still have warranty and, even if not, maybe some persistence will pay off. I can say that the service process under warranty was free, smooth, fast (I think they promised 5 business days) and I was fully informed at each step. I could have had FedEx pick it up but I chose to take it to a FedEx shipping point; the return was to my door.
