I am aware this is an OLD thread, but as I was suffering from exactly the same issue and did not yet find a solution here, I am posting it here for posterity and my good self.
Got a PiFi Digi+ Pihat too today and was struggling to get it to work on a Raspberry PI 3 running ** RasPlex (official) Version: 1.8.0.148-573b6d73 **
what made it tick for me were the following steps:
open an ssh console, login as root:rasplex
make /flash writable (mount -o remount,rw /flash)
open /flash/config.txt (nano -w /flash/config.txt) and edit. The IR eye (dtoverlay=lirc-rpi) seems to bork the correct initialization of the soundcard. as soon as I commented that out; optical out (spdif)started working. So find the line that enables “dtoverlay=lirc-rpi” and comment it out by placing a # in front of it.