Use SDL_LockAudio/SDL_UnlockAudio as suggested by the docs.

(Shouldn't actually be necessary, but oh well).

src/xpdev/sdlfuncs.c

@@ -56,6 +56,8 @@ int load_sdl_funcs(struct sdlfuncs *sdlf)
 	sdlf->OpenAudio=SDL_OpenAudio;
 	sdlf->CloseAudio=SDL_CloseAudio;
 	sdlf->PauseAudio=SDL_PauseAudio;
+	sdlf->LockAudio=SDL_LockAudio;
+	sdlf->UnlockAudio=SDL_UnlockAudio;
 	sdlf->gotfuncs=1;
 	sdl_funcs_loaded=1;
 	return(0);
@@ -211,6 +213,14 @@ int load_sdl_funcs(struct sdlfuncs *sdlf)
 		FreeLibrary(sdl_dll);
 		return(-1);
 	}
+	if((sdlf->LockAudio=GetProcAddress(sdl_dll, "SDL_LockAudio"))==NULL) {
+		FreeLibrary(sdl_dll);
+		return(-1);
+	}
+	if((sdlf->UnlockAudio=GetProcAddress(sdl_dll, "SDL_UnlockAudio"))==NULL) {
+		FreeLibrary(sdl_dll);
+		return(-1);
+	}
 	sdlf->gotfuncs=1;
 	sdl_funcs_loaded=1;
 	return(0);
@@ -360,6 +370,14 @@ int load_sdl_funcs(struct sdlfuncs *sdlf)
 		dlclose(sdl_dll);
 		return(-1);
 	}
+	if((sdlf->LockAudio=dlsym(sdl_dll, "SDL_LockAudio"))==NULL) {
+		dlclose(sdl_dll);
+		return(-1);
+	}
+	if((sdlf->UnlockAudio=dlsym(sdl_dll, "SDL_UnlockAudio"))==NULL) {
+		dlclose(sdl_dll);
+		return(-1);
+	}
 	sdlf->gotfuncs=1;
 	sdl_funcs_loaded=1;
 	return(0);

src/xpdev/sdlfuncs.h

@@ -42,6 +42,8 @@ struct sdlfuncs {
 	void (*QuitSubSystem)(Uint32 flags);
 	int (*OpenAudio)(SDL_AudioSpec *desired, SDL_AudioSpec *obtained);
 	void (*CloseAudio)(void);
+	void (*LockAudio)(void);
+	void (*UnlockAudio)(void);
 	void (*PauseAudio)(int pause_on);
 	int	gotfuncs;
 };

src/xpdev/xpbeep.c

@@ -427,11 +427,13 @@ BOOL DLLCALL xptone(double freq, DWORD duration, enum WAVE_SHAPE shape)
 
 #ifdef WITH_SDL
 	if(handle_type==SOUND_DEVICE_SDL) {
+		sdl.LockAudio();
 		sdl_audio_buf_pos=0;
 		sdl_audio_buf_len=S_RATE*duration/1000;
 		if(sdl_audio_buf_len<=S_RATE/freq*2)
 			sdl_audio_buf_len=S_RATE/freq*2;
 		makewave(freq,swave,sdl_audio_buf_len,shape);
+		sdl.UnlockAudio();
 		sdl.PauseAudio(FALSE);
 		sdl.SemWait(sdlToneDone);
 		sdl.PauseAudio(TRUE);