Move sound.c into the sound system source and rename to sdl.c.

The OpenAL backend was already part of the sound system source. And
there's no need for the SDL backend to be part of the generic backends.

CMakeLists.txt

@@ -168,7 +168,6 @@ set(Backends-Generic-Source
 
 	${BACKENDS_SRC_DIR}/sdl/input.c
 	${BACKENDS_SRC_DIR}/sdl/refresh.c
-	${BACKENDS_SRC_DIR}/sdl/sound.c
 	)
 
 set(Backends-Generic-Header
@@ -322,6 +321,7 @@ set(Client-Source
 	${CLIENT_SRC_DIR}/sound/ogg.c
 	${CLIENT_SRC_DIR}/sound/openal.c
 	${CLIENT_SRC_DIR}/sound/qal.c
+	${CLIENT_SRC_DIR}/sound/sdl.c
 	${CLIENT_SRC_DIR}/sound/sound.c
 	${CLIENT_SRC_DIR}/sound/wave.c
 	${COMMON_SRC_DIR}/argproc.c

Makefile

@@ -694,7 +694,6 @@ CLIENT_OBJS_ := \
 	src/backends/generic/vid.o \
 	src/backends/sdl/input.o \
 	src/backends/sdl/refresh.o \
-	src/backends/sdl/sound.o \
 	src/client/cl_cin.o \
 	src/client/cl_console.o \
 	src/client/cl_download.o \
@@ -715,6 +714,7 @@ CLIENT_OBJS_ := \
 	src/client/menu/menu.o \
 	src/client/menu/qmenu.o \
 	src/client/menu/videomenu.o \
+	src/client/sound/sdl.o \
 	src/client/sound/ogg.o \
 	src/client/sound/openal.o \
 	src/client/sound/qal.o \