FYI: I tried Roon for a week. In Roon the back / next media keys of the Hegel work.
This is on a Mac mini mid 2011, OSX high Sierra. With Audirvana 3.5.44
Just an update. I recently updated my computer to macOS 11 (Big Sur) - and now not even the play / pause media key works with Audirvana (the only one that worked before) 
Hello @glawrie,
Can you open your system preference, in the keyboard section, can you select the tab “Input Source” and select the option: Show Input menu in menu bar? After doing this in the menu, select “show keyboard viewer”. Now start an album and try to switch from a track to another with the keyboard viewer, does it works?
Hello. No - what happens is that when the keyboard media keys are pressed the computer opens up the Music app (I guess at some level it is the default if nothing else is playing?). Here is a link to a video that shows this in operation (if you need to see it).
HTH
What is the output you have selected in Audirvana?
Are you connected to it via USB or Jack?
Okay, in the MM-1 settings in Audirvana, do you have Exclusive Access enabled?
Can you enable it and try to play your track in Audirvana using the keyboard viewer?
It’s really strange as when we try on our Mac with BS 11.2.3 we don’t have this issue.
I have no suggestions. As you can see it is happening here, but why is a mystery to me. As per the start of this now very mature thread, the media keys for advance / back have not been working for a long time, the only change now is that play / pause is also not working.
I hope you can use this information to guide your development such that the problem is fixed - I am used to using the media keys to pause / restart the player while working in other apps - having to seek out the app and control playback from there is not the end of world, but unnecessary and a bit irritating.
Hello @glawrie,
Can you go in System Preferences in Security&Privacy and in the Privacy check the Accessibility section. Does Audirvana is enabled?
If you disable it and reopen Audirvana, do you have the window that ask you to enable Audirvana in media keys?
Yes
I disabled and the option, closed Audirvana, and then re-enabled the option.
After this the play / pause mediakey now works!
In these diminshed times, simply getting this to work again is a big step forward.
The ‘forward / back’ media keys on keyboard still do not work, but for the first time they are now working OK via the keyboard simulator.
To check the keyboard issue, I restarted my computer, but the outcome was not changed.
I will do some investigation with Logitech on whether there is something weird happening with how it sends the forward / back keystrokes to apps - they work with other apps, so I’m guessing not, but maybe they have some ideas.