From Serge Lages, "Here is a patch to allow setting an audio volume with the AudioSink interface, I've also modified the ffmpeg plugin code to implement the ImageStream's setVolume method with its AudioSink."

src/osgPlugins/ffmpeg/FFmpegDecoderAudio.cpp

@@ -112,6 +112,22 @@ void FFmpegDecoderAudio::close(bool waitForThreadToExit)
     }
 }
 
+void FFmpegDecoderAudio::setVolume(float volume)
+{
+    if (m_audio_sink.valid())
+    {
+        m_audio_sink->setVolume(volume);
+    }
+}
+
+float FFmpegDecoderAudio::getVolume() const
+{
+    if (m_audio_sink.valid())
+    {
+        return m_audio_sink->getVolume();
+    }
+    return 0.0f;
+}
 
 void FFmpegDecoderAudio::run()
 {

src/osgPlugins/ffmpeg/FFmpegDecoderAudio.hpp

@@ -34,6 +34,9 @@ public:
     void pause(bool pause);
     void close(bool waitForThreadToExit);
     
+    void setVolume(float volume);
+    float getVolume() const;
+
     virtual void run();
 
     void setAudioSink(osg::ref_ptr<osg::AudioSink> audio_sink);

src/osgPlugins/ffmpeg/FFmpegImageStream.cpp

@@ -152,6 +152,15 @@ void FFmpegImageStream::quit(bool waitForThreadToExit)
     m_decoder->close(waitForThreadToExit);
 }
 
+void FFmpegImageStream::setVolume(float volume)
+{
+    m_decoder->audio_decoder().setVolume(volume);
+}
+
+float FFmpegImageStream::getVolume() const
+{
+    return m_decoder->audio_decoder().getVolume();
+}
 
 double FFmpegImageStream::getLength() const
 { 

src/osgPlugins/ffmpeg/FFmpegImageStream.hpp

@@ -33,6 +33,9 @@ namespace osgFFmpeg
         virtual void seek(double time);
         virtual void quit(bool waitForThreadToExit = true);
 
+        virtual void setVolume(float volume);
+        virtual float getVolume() const;
+
         virtual double getLength() const;
         virtual double getReferenceTime () const; 
         virtual double getFrameRate() const;