I'm having the same problem as the original poster. Sound is fine until fast-forwarding when display is set to 4k. No issues at all when set to 1080p. Cables are not the issue, as I've tried multiple HDMI 2.1 cables.
Roku Ultra (4660X)
Software version 9.2.0 (build 4750-46)
I have experienced the same problem with the newly released HDR Roku. I have a LG TV and I think what you need to do is go into the picture settings for HDR and play around with what is being output. I noticed that when I downgraded the 4K content the atmos worked without being lost as soon as you used the fast forward to reimwind. Let me know by emailing me on [personal information removed] if this trick works? Sorry not sure if I'll be able to find this page again to see someone's reply. But I'm curious to see of this resolves the issue. Est of luck folks.
I am experiencing this issue but in my TCL 4k Roku tv and into my onkyo RZ720 AVR with a 5-1-1 speaker set up. i get no atmos at all. I get dolby vision HDR or so the screen badge shows whenever i play that content. i can play DD+ in a 7.1 speaker configuration with THX Select and Dolby surround sound out of my 7.1. just in the dolby atmos streaming movies in 4k i have all speaker channels silent unless i select all channels stereo , mono or the theater 3.1 they each get audio but no atmos. in my 5.1.2 speaker config. i dont know if this is a roku issue or not. i can not get answers from anyone but amazon prime video. they offer suggestions for only the fire tv and prime, sent me here or to roku for prime video help. so far i have not gotten anything and this is the first ive seen with similar issues though it has been over 8months since i last checked. there are so few dolby atmos /HDR movies to pick from, i watched netflix last year for 4weeks and saw everything i wanted to so dropped it. i did get hdr and atmos there in the few netflix originals with that high def audio and vid. i m still seeking a fix and cant pin down the culprit or conflicting issues. by the way. i have all brand new sets of HDMI cabling with the highest specs for 3D, hdr, atmos and 18Gbps , the highest level and i got 2 or 3 sets over the past 5 or 6months in case i got duds or the specs had been rated higher than they actually provided. i used amazon and new egg and best buy for these 4k HDR Dolby vision capable cables . all provide the same end result . no Atmos. every thing else though.
i have Roku Ultra hooked right up to samsung HW-K950 and it used to play atmos but no longer.
what is the deal???
I swapped HDMI cables to no avail.
i am using VUDO with ATMOS titles (hellboy 2, spiderman homecoming, etc.) and it is in 4k but no atmos.