From my brief surfing as far as I understand it the Chromecast Ultra is your problem as it does not appear to support DTS.
For the record I have many mkv files with DTS audio that I stream via DLNA to devices and have no problems.
I have DVD/Blue Ray ripped to MKV.
On Network DLNA playback, my Core i5 CPU maxes out to 100% and the playback is very slow/jerky/stops and audio is extremely intermittent. All playback as Dolby Digital 5.1- no DTS.
Is there anyway this can be eradicated? OR may be improved to watchable?
How are you streaming? Can you help please.
Regards
Setup: DVD/Blue Ray ripped MKV & stored in HD connected to Laptop. Video sent via Serviio media server to Panasonic BDT110 (using Roku-non 4kTV profile-displaying on Samsung UE55JU7000 TV.