I haven't tried any Qt4.6 development but have managed to get python to monitor mic audio levels using gstreamer tools

pipeline = gst.parse_launch("pulsesrc ! level message=true ! fakesink")
bus = pipeline.get_bus()
bus.enable_sync_message_emission()
bus.add_signal_watch()
bus.connect("message::element", on_message)
pipeline.set_state(gst.STATE_PLAYING)

then create a callback function that acts on the message retuned from the level monitor (you can control the frequency of the level sampling too)