diff --git a/reactos/drivers/wdm/audio/backpln/portcls/dma_slave.c b/reactos/drivers/wdm/audio/backpln/portcls/dma_slave.c
index 7e37c5d70c3..73977b82cd4 100644
--- a/reactos/drivers/wdm/audio/backpln/portcls/dma_slave.c
+++ b/reactos/drivers/wdm/audio/backpln/portcls/dma_slave.c
@@ -42,6 +42,7 @@ typedef struct
 // IUnknown methods
 //
 
+extern GUID IID_IDmaChannelSlave;
 
 NTSTATUS
 NTAPI
diff --git a/reactos/drivers/wdm/audio/backpln/portcls/port_wavecyclic.c b/reactos/drivers/wdm/audio/backpln/portcls/port_wavecyclic.c
index 5ddb338149b..c2005238fcf 100644
--- a/reactos/drivers/wdm/audio/backpln/portcls/port_wavecyclic.c
+++ b/reactos/drivers/wdm/audio/backpln/portcls/port_wavecyclic.c
@@ -8,6 +8,8 @@
 
 #include "private.h"
 
+extern GUID IID_IDmaChannelSlave;
+
 typedef struct
 {
     IPortWaveCyclicVtbl *lpVtbl;