See p.29 in the user manual online. ‘Connection notes for audio devices’
Make sure to select Optical.
I bought a samsung q70r and I've tried setting up my onkyo HT- R494. The tv recognises the sound system through arc and even comes up with the name but there is no sound. I've tried enabling/disabling atmos but still no sound. I've also tried using optical and plugged my ps4 to receiver but still nothing. I've tried a sound test on the system itself and all the speakers are working fine. It also worked fine on my last older tv the sony xe90. I called samsung and they were useless and basically said the sound system was too old for the tv. Does anyone know how I can fix this? I've inclused a link to the tv and sound system below, thanks.