The version of multimedia/syncplay in the ports tree is unusable because it's incompatible with the version of MPV in the ports tree from which it depends on for core functionality. When trying to launch the software it errors with the message: "Syncplay is not compatible with this version of mpv. Please use a different version of mpv (e.g. Git HEAD)." Running the latest syncplay from git revision 9b53026db7061aa1a1022d60d067d424522e3da1 resolves the issue.