After a few years of frustration I have found a solution. Under System Preferences/Keyboard/Touch Bar Shows, select: F1, F2, etc. Keys. Then using the “fn key” you get your volume buttons and they work for Audirvana.
I have not been able to find where I can get it to move by only 1 dB tho. If anyone finds that out please let me know. I am using an external Schiit Audio Gunginer Multibit DAC and while waiting on my preamp I am using Audirvana for volume control. My systems balanced signal is up there so I am in the very low -45 to -20 dB and have to be very careful with my volume setting. Thankfully I just figured this out.
I have done searches on this and found no posting so I though I would share.