Xandr/AppNexus is your first and primary source of support for any Xandr/AppNexus related issues. However, the Triton Digital Help Center contains a few items that might help you with some basic problems if you are an AppNexus user. But AppNexus documentation and support always take precedence over any AppNexus articles found in the Triton Digital Help Center.
If "Audio" does not appear as a "Media Type" option on the Creative Content tab of the Edit Creative page in Xandr/AppNexus, you need to contact Xandr/AppNexus Technical Support and request that they activate audio for you.