You're creating 2 separate pipelines reading the same file, one GstDiscoverer pipeline to get the tags, one playbin pipeline to reading.

Even though GstDiscoverer is relatively lightweight, it's still a separate pipeline, doing its own reading, parsing, etc. The tags are already available from playbin.

Other problems with the current code include:
- using "playbin" instead of the better supported, and with less bugs "playbin2"
- Absolute horrors like:
 pipeline_desc = g_strdup_printf("playbin uri=\"%s\"",
                                  priv->uri);
(Hint, it's a GObject, why would you marshal the uri to a string, only to have GStreamer demarshal it to a separate string)
play = gst_element_factory_make ("playbin2", "play");
g_object_set (play, "uri", priv->uri);

Given that this code has an exception for proprietary GStreamer plugins (as Totem does), some basic code could be taken from totem-audio-preview.c and the likes.