This may be more a question for Emscripten/WASM than SDL, but I’m finding that SDL_ShowSimpleMessageBox() is blocking sounds played using SDL_QueueAudio(), the result being an unpleasant looping of the most recently transferred buffer. On other platforms this doesn’t happen and sound which has already been queued continues to play whilst the message box is displayed.
SDL_ShowSimpleMessageBox() blocks the current thread.
On other platforms, audio is managed on a separate thread, so it’s no problem. But IIRC on Emscriptem/WASM you only get one thread, so the audio buffers etc don’t get updated until the message box closes.
Fortunately that’s no longer true (my app is multi-threaded and works, on some browsers anyway) so the audio could perhaps (now) be handled in a web worker.