Thank you for the profile link. That helped me immensely, not only with Serviio, but it helped pinpoint issues I have been having with other media servers I use with my Hopper.
I do have one minor tweak to suggest for users who may still have issues with audio files.
The Hopper seems to be very sensitive to bitrates, and many of the mp3 podcasts I was streaming weren't playing properly.
In order to resolve that, I added the line below to the Audio section to force transcoding of MP3 files:
<Matches container="mp3"/>
Now all audio files are playing as expected. Thanks again!