Audacious has finally MPRIS 2 support so it's possible to use it with e.g. media player indicator for gnome shell (https://extensions.gnome.org/extension/55/media-player-indicator/). Thanks for that!
It seems, that currently the volume and the seeking bar are not supported by audacious, so the song length is.
Are there any plans to implement these missing features?
You can add a feature request for this.
Added a feature request: http://redmine.audacious-media-player.org/issues/58