In the metadata detection field I uploaded the xml file. I've tried two xml files: one that previously worked and a new one. After hitting 'submit' both didn't go through and gave the same error message (as mentioned above).

Dear all,

Ah, mczusatz.
Then I would still appreciate if soundcloud could be whitelisted as a domain in its entirety. Much like flickr for images it can function as a portal between gwtoolset users and wikimedia commons for sound.
Of course as a user of the toolset ill make sure to consult the community before performing the actual upload.

I appreciate Krenair's comment, but since flickr is also whitelisted (which is basically the other half of the web ;-) I figured...