I don't have a Kobo, but, AFAIK, regular epub3 books might need to be converted to
KEPUB files for use with Kobo readers and apps.
Since there doesn't seem to be anything wrong with your test file, you might want to download the semiofficial
Creative Commons Shared Culture IDPF audio/video sample file and test it with the Kobo app. If it doesn't work either, it's definitely a Kobo app issue.