[jjd-uk]

Unless the Pi does things differently, then as it's a HDMI output going to a Optical SPDIF connection, then the HDMI device in Kodi settings must be set to Number of Channels 2.0 (as the Optical is the bottleneck limited to 2 channels, note that's the number of uncompressed PCM channels available) then make sure the Settings Level is set to Advanced or Expert to see all the passthrough options in order to make sure you have enabled "Dolby Digital (AC3) capable receiver" and "DTS capable receiver".

In addition if you have anything that decodes to Multichannel PCM such as flac or aac 5.1 then "Enabled Dolby Digital transcoding" must also be selected for that Multichannel PCM can be encoded into compressed Dolby Digital to maintain the 5.1 channels.