I have no idea what I did, but it just started working out of nowhere and Volumio is currently scanning my music library. For the most part I followed what domchocolate said.
I rejiggered my network and Mac a bit over the weekend so that may have something to do with it. For reference, when I set up my static IP, under “Configure IPv4” I used “Using DHCP with manual address” and then set my manual address. My MacPro is also connected via Ethernet but my Pi is connected via wifi. Not sure if any of this makes any difference but it is working for me now.
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