diff --git a/src/libs/decoders/Makefile.am b/src/libs/decoders/Makefile.am
index adba928e..80ad40ef 100644
--- a/src/libs/decoders/Makefile.am
+++ b/src/libs/decoders/Makefile.am
@@ -1,3 +1,5 @@
+AM_CPPFLAGS = -I$(top_srcdir)/include
+
 noinst_LIBRARIES = libdecoders.a
 
 libdecoders_a_SOURCES = \
@@ -33,6 +35,7 @@ libdecoders_a_CFLAGS = \
 	-Wall
 
 if USE_OPUS
-libdecoders_a_SOURCES += opus.c
-libdecoders_a_CFLAGS += -DUSE_OPUS $(OPUSFILE_CFLAGS)
+libdecoders_a_SOURCES += opus.cpp
+libdecoders_a_CFLAGS += -DUSE_OPUS # Ensures Opus is registered in SDL_sound.c
+libdecoders_a_CXXFLAGS += $(OPUSFILE_CFLAGS) # Ensures opus.cpp find the header
 endif
diff --git a/src/libs/decoders/opus.c b/src/libs/decoders/opus.cpp
similarity index 100%
rename from src/libs/decoders/opus.c
rename to src/libs/decoders/opus.cpp
diff --git a/vs/dosbox.vcxproj b/vs/dosbox.vcxproj
index 70881533..06b99e05 100644
--- a/vs/dosbox.vcxproj
+++ b/vs/dosbox.vcxproj
@@ -247,7 +247,7 @@
     <ClCompile Include="..\src\libs\decoders\flac.c" />
     <ClCompile Include="..\src\libs\decoders\mp3.cpp" />
     <ClCompile Include="..\src\libs\decoders\mp3_seek_table.cpp" />
-    <ClCompile Include="..\src\libs\decoders\opus.c" />
+    <ClCompile Include="..\src\libs\decoders\opus.cpp" />
     <ClCompile Include="..\src\libs\decoders\SDL_sound.c" />
     <ClCompile Include="..\src\libs\decoders\vorbis.c" />
     <ClCompile Include="..\src\libs\decoders\wav.c" />