The call option only shows during AppleCare's US business hours, from what I know, there might be timezone differences. Would suggest just emailing them and they could schedule a callback.

If anyone is here regarding specifically visionOS and is on a Intel mac, I would like to note that the current release notes say they don't support Intel macs anymore currently.

I had a similar question and asked this during a WWDC Lab. Essentially, the answer was that only applications running in the full space could directly display Metal content. However, RealityKit allows custom materials with Metal, similar to Unity URP, which may suit your needs. While not applicable to this context, you can also indirectly display Metal Content thru ARKit Custom Views, if needed.

I'm interested in lossless audio in MusicKit JS. I would encourage anyone else interested submit feedback in Feedback Assistant. I just did in FB11761886.