I’m pretty new to this having just received my first pi (2b) yesterday, but have everything set up and working perfectly (thanks to OSMC for making this a breeze!) …
… apart from one thing - DTS passthrough audio works great until I seek (fastforward, rewind, skip), then it goes silent until I fully stop/exit the video and resume playback that way. The DTS indicator on my amp stays there, so it doesn’t look like the DTS connection is fully dropped.
Pausing/resuming the video is fine, DTS audio comes back after a short delay.
Dolby Digital doesn’t have this problem, I can skip around and not lose sound.
I’m currently going via HDMI to the TV, then coax to the amp. If I can’t get this working, then maybe hifiberry would let me go via optical straight to the amp for better results? I’d rather not go that route if I don’t have to though, as case choice would be limited and I’m not currently interested in any of the extra DAC sound benefits.
I suspect it may be something your TV is doing (passing DTS from HDMI to optical is actually quite rare for a TV to support).
Can you try disabling DTS passthrough. Make sure number of channels is set to “2.0”. Enable AC3 passthrough and “AC3 transcode”. You’ll hopefully now get multichannel audio from your DTS files (but output as AC3), and that may avoid the silence after seek issue.
Doing that only gave me 2 channel audio, but searching for others having trouble with ac3 transcode led me to this thread: 5.1 DTS -> 5.1AC3 - only stereo
Turning off omxplayer solved the ac3 transcoding problem, and actually solved DTS passthrough as well! Not sure exactly what I’m missing without omxplayer, but too happy with working sound to care right now 
P.s. for the record, it’s digital coax from my TV to the amp, not optical
Oh, that’s what omxplayer does … I’m skipping frames with it switched off 
All working now, switched omxplayer off and enabled ‘Adjust display refresh rate’ AND ‘sync playback to display’. Smooth playback, no dropped frames and passthrough DTS/AC3 that doesn’t drop out when seeking.