Simutrans stopped playing any sounds around SVN revision 8559 with the SDL2 backend on my computer.
According to this question (https://discourse.libsdl.org/t/no-sound-using-sdl-audio/23839) on the SDL forum, it is related to mixing legacy SDL functions with newer ones.
The attached patch solved the problem for me.
Incorporated in r8594, thanks. I had sound before, so please test it for you.