playbin and volume in C++

[Marco Trapanese]

Hello,
from command line I type:

gst-launch-1.0 playbin uri=file:///path-to-mp4 audio-sink=alsasink 
volume=1.0

and it just works.
Now I want to do the same from a C++ application:

GstElement *pipeline;
GstElement *audiosink;
GstElement *volume;

pipeline = gst_element_factory_make("playbin", "player");
g_object_set(G_OBJECT(pipeline), "uri", "path-to-mp4", NULL);

volume = gst_element_factory_make("volume", "volume");
g_object_set (G_OBJECT(pipeline), "volume", volume, 1.0, NULL);

audiosink = gst_element_factory_make ("alsasink", "audio_sink");
g_object_set (G_OBJECT(pipeline), "audio-sink", audiosink, NULL);

gst_element_set_state(GST_ELEMENT(pipeline), GST_STATE_PLAYING);


But the audio doesn't work. If I remove the two "volume" lines it works.
Where is the mistake?

When the pipeline is playing will I be able to change the volume just 
calling again the g_object_set with another volume value?

Thanks!
Marco