Hooray! Yes thanks that worked out well. I had to log in twice - once as guest to the network and then once as the piCore user with the password to the shares.OK, found it. I installed picoreplayer on a RPI 3B+.
At the top of smb.conf (/usr/local/etc/samba/smb.conf) you need an additional 2 lines after workgroup:
workgroup = WORKGROUP
lanman auth = yes
ntlm auth = yes
client min protocol = NT1
server min protocol = NT1
I found on a Raspbian install you also need those two additional lines.
The Oppo played the surround
FLACS through the network, but would not play .mkv and did not recognize .mka files/