Bang, Iccuslus suddenly exited from hibernation and fixed sdl_sound... ! See there : https://github.com/icculus/SDL_sound/commits/main almost 1 month later, but better now than never of course...
Now I hesitate : is it better to remove the new mp3 dll and return to the old decoder and just forget all that and stay with what we have ?
Well luckily only 1 version used the new dll, and it was included in the raine archive, so removing it now will have no consequence.
So I guess it's probably the thing to do, the mp3 decoder used in sdl_sound is still smaller and maybe faster...
Oh at least I'll have to try it first. Apparently the bug was that when the decoder returned something less than the expected bytes to read (which happens for the last block), it was understood as eof (end of file), and so the last block was not handled. Oh well... He finally closed the bug thread on github with this fix.
edit : yeah, the fix works !