By "both devices" you mean the speaker or soundbar too, right?
Absolutely no audio at all- even old mono stuff or- say- only ATMOS video but other video has audio?
Took some time to check with my own setup: AppleTV to Receiver to Samsung TV. I've already upgraded to 17.5 so I can't be sure about the "before" but I "THINK" that when I used to force the Receiver into TV Audio mode (instead of AppleTV mode), AppleTVs audio would basically go to TV and then come back to Receiver to play on speakers. Now it IS silence when I do that. But I'm not sure about it working like that in the past.
I tried altering a few settings in the TV and then AppleTV to see if I could get any "audio return" sound sourced from AppleTV playing and nothing worked, even with ATMOS options off and forcing Dolby Digital instead. So I put everything back to how it normally is and feel greater suspicion that 17.5 did do something to eArc.
I cut AppleTV out of this connection by trying local over-the-air channels on the Samsung (receiver still in TV mode) and audio played normally. Next I'm going to try my ONE other video source to Receiver to Samsung TV to see if video will play (and be heard) from it (which seems like it would isolate this to AppleTV).
Second source would also NOT play audio when Receiver was switched to TV mode (thus leaning on audio return channel). This makes me believe this never worked like that and the only time I've ever had audio work in TV mode is when watching audio sourced from the TV itself: over-the-air, TV's own apps, etc.
With 2 distinct devices not "passing through" audio, I'd shift a little blame OFF AppleTV. However, OP and I also have almost certainly DIFFERENT Samsung TVs, so it's probably not his TV, cable, etc either. Others leaning on AppleTV to Samsung TV to audio via eArc need to chime in to help this get somewhere.
What is likely different for me vs. OP is my first connection from AppleTV and other source is Receiver: video source to receiver (which them splits video & audio) to Samsung & speakers... while I think OP is AppleTV to Samsung to Speakers or maybe a soundbar. Others with what is probably OPs setup needs to confirm eArc still works for them (or not) after 17.5 update.
I just tried bypassing that Receiver as 2nd link in the chain and connecting AppleTV direct to Samsung, which would force the audio return channel to work if there is to be sound: AppleTV to Samsung to Receiver. That setup which has to have Receiver in "TV mode" had no trouble at all with all types of audio, including ATMOS audio. My new guess is that in the normal way I'm connected, the Receiver "keeps" the audio signal and ONLY sends the video over to the TV... thus TV mode has no audio to return from TV.
OP, sorry I couldn't be more helpful. The one thing you might try is a slight variation of what I just did: unhook the HDMI cable from the current HDMI jack on your Samsung and try another. While unlikely to fix your issue, it's easy enough to try that.
And, of course, try some other sources to narrow in on it being AppleTV vs. something else. If other things play just fine and only AppleTV is not, the problem is much more likely AppleTV. Even use the SAME cable hooked to the same port to rule out those few loose ends. For example, test a game console or cable box or Mac using the very same (AppleTV) HDMI cable and very same port on the Samsung. If they play audio as you expect it from AppleTV, the problem is towards certainly AppleTV. Since AppleTV played audio just fine before 17.5, 17.5 would seem to have altered something that created this problem.