I set up my environment with FFmpeg4.2 & SDL2.10, and the environment is OK since my own code can run. However, I run into the same problem with you and want to run the code in FFmpeg-playAudio.cpp but it don't work.
I have solved this issue in my fork, by simply replace the SDL_MixAudio() with raw memcpy, though I still don't know why SDL_MixAudio cannot work with SDL 2.10.
I set up my environment with FFmpeg4.2 & SDL2.10, and the environment is OK since my own code can run. However, I run into the same problem with you and want to run the code in FFmpeg-playAudio.cpp but it don't work.