Further info. Although I can now play videos with DTS-HD MA tracks successfully, normal music tracks in 5.0 format are silent in the rear channels again. Also, after playing video with tracks containing high-res audio and then returning to the music player all normal 2.0 audio tracks are silent (and navigation clicks are silent) until I play a high-res multichannel audio track first. Then, 2.0 audio will play and navigation sounds are enabled. Similar issue happened way back in 19.4 Matrix builds and fixed by @Portisch - see here https://discourse.coreelec.org/t/coreelec-19-4-matrix-discussion/18094/110