GstVideoUtils setVolume error custom pipe

I work on a custom gst pipe to play videos with audio output to jackd. (ubuntu)

here works well enough for easy reading.
I would like to change the volume of ofGstVideoUtils player with gst.setVolume (float) this is the message received:

  GLib-GObject-WARNING **: g_object_set_valist: object class 'GstPipeline' has no property named 'volume' 

is that you have an idea?