diff --git a/demos/second/part15_transmission/Makefile b/demos/second/part15_transmission/Makefile
index b67de533..eea1ac56 100644
--- a/demos/second/part15_transmission/Makefile
+++ b/demos/second/part15_transmission/Makefile
@@ -19,6 +19,7 @@ spheres.o:	spheres.s \
 	../hgr_copy_fast.s \
 	../audio.s \
 	../zp.inc ../hardware.inc ../qload.inc \
+	audio/transmission.btc.zx02 \
 	graphics/spheres.hgr.zx02
 	ca65 -o spheres.o spheres.s -l spheres.lst
 
@@ -27,9 +28,12 @@ spheres.o:	spheres.s \
 graphics/spheres.hgr.zx02:	
 	cd graphics && make
 
+audio/transmission.btc.zx02:
+	cd audio && make
 ###
 
 
 clean:
 	rm -f *~ *.o *.lst SPHERES
 	cd graphics && make clean
+	cd audio && make clean