Quick review of the rygel.pot file:

#: ../data/rygel-preferences.ui.h:1
msgid "<b>Network Options</b>"
Please avoid using markup if possible. See "Avoid markup wherever possible" under http://live.gnome.org/TranslationProject/DevGuidelines

#: ../data/rygel-preferences.ui.h:11
msgid "T_racker"
A bit unclear what it means in this context. See http://live.gnome.org/TranslationProject/DevGuidelines/Use%20comments

#: ../src/plugins/media-export/rygel-media-export-plugin.vala:33
#: ../src/plugins/tracker/rygel-tracker-plugin.vala:33
msgid "@REALNAME@'s media"
Please add a comment what @REALNAME@ is and that it should not be translated. See http://live.gnome.org/TranslationProject/DevGuidelines/Use%20comments

#: ../src/plugins/media-export/rygel-media-export-query-container.vala:47
msgid "Id does not contain pairs"
#: ../src/plugins/media-export/rygel-media-export-query-container.vala:54
msgid "Empty part not allowed in virtual id"
#: ../src/plugins/media-export/rygel-media-export-root-container.vala:346
#, c-format
msgid "%s is not a valid virtual id"
#: ../src/plugins/media-export/rygel-media-export-root-container.vala:373
#, c-format
msgid "Id %s no longer in config, deleting..."
#: ../src/rygel/rygel-media-db.vala:292 ../src/rygel/rygel-media-db.vala:318
#, c-format
msgid "Failed to add item with id %s: %s"
Id should be ID.

#: ../src/plugins/media-export/rygel-media-export-query-container.vala:118
#, c-format
msgid "We have %u children"
This needs ngettext support. See http://live.gnome.org/TranslationProject/DevGuidelines/Plurals

#: ../src/plugins/media-export/rygel-media-export-root-container.vala:97
#, c-format
msgid "Failed to remove uri: %s"
uri should be URI, as before.

#: ../src/plugins/mediathek/rygel-mediathek-asx-playlist.vala:94
msgid "Could not received XML"
Typo? "receive"?

#: ../src/plugins/mediathek/rygel-mediathek-video-item.vala:64
msgid "group node has url property"
url should be URL

#: ../src/plugins/tracker/rygel-tracker-category-all-container.vala:38
#, c-format
msgid "Failed to contstruct URI for directory '%s': %s"
Typo: contstruct -> construct

#: ../src/plugins/tracker/rygel-tracker-item-creation.vala:77
#: ../src/rygel/rygel-media-container.vala:180
#, c-format
msgid "Object creation in %s no allowed"
no -> not ?

#: ../src/plugins/tracker/rygel-tracker-metadata-values.vala:75
#, c-format
msgid "Failed to create to Session bus: %s"
The "to" is confusing here. If it is correct maybe at a translation comment.

#: ../src/rygel/rygel-browse.vala:120
msgid "Invalid Args"
"Arguments" for the sake of less confused translators? :)

#: ../src/rygel/rygel-database.vala:217
#, c-format
msgid "Failed to rollback transaction: %s"
If it's a verb it should be "roll back"?

#: ../src/rygel/rygel-live-response.vala:83
msgid "Plugin 'fakesink' missing"
Worth to mention in a comment that fakesink should not be translated.

#: ../src/rygel/rygel-media-db.vala:452
#, c-format
msgid "Orignal search: %s"
Typo: Orignal

#: ../src/rygel/rygel-metadata-extractor.vala:116
msgid "Could not find any playbin. "
Please remove the whitespace at the end.

#: ../src/rygel/rygel-root-device-factory.vala:102
msgid "Element /root/device not found."
#: ../src/rygel/rygel-root-device-factory.vala:135
msgid "Element /root/device/friendlyName not found."
#: ../src/rygel/rygel-root-device-factory.vala:156
msgid "Element /root/device/UDN not found."
#: ../src/rygel/rygel-root-device-factory.vala:175
msgid "Element /root/device/modelDescription not found."
#: ../src/rygel/rygel-root-device-factory.vala:189
msgid "Element /root/device/serviceList not found."
Can these be turned into "Element %s not found." ?

#: ../src/rygel/rygel-seekable-response.vala:86
#, c-format
msgid "Failed to seek to %s-%s on URI %s: %s"
Translation of "to" depends on what %s-%s is. Please add a translator comment.

#: ../src/rygel/rygel-user-config.vala:344
#, c-format
msgid "Failed to %s Rygel service: %s"
Untranslatable without knowing what the first %s is. Please add a translator comment.

In general:
http://live.gnome.org/DocumentationProject/Terminology/RoughList states that "folder" is prefered to "directory/directories".