I have a DS 3615xs that is primarily dedicated to my Plex worlld. It has more than enough horse power to handle several streams of UHD video and audio should never be an issue. However the latest build (9.15.0) is causing some poor audio quality issues. A bit of a crackle sound. I confirmed this to be Plex by playing a regular audio CD of the same song. I also ruled out that my Synology, Roku 4, Denon AVR-X7200WA are not a fault by play music from the Synology Audio Station directly to my Denon AVR-X7200WA, playing streaming music from Roku 4 to my Denon AVR-X7200WA.
Basically by passing Plex all together. But every time I play music using Plex the quality drops tremendously and is some case so bad it starts to crackle.
My Audio path starts with the Synology DS 3615xs then my Roku 4 with the Plex app, then HDMI audio to my Denon AVR-X7200WA to output to my Polk LSiM707s.
If I recall correctly Plex has introduced a new feature that is to optimize the stream based on the connection type and/or bandwidth requirements. This is great for remote devices, but please leave us with manual control for local configurations. Not sure if Media Optimizer is for audio or not, but none the less something has changed that is causing this issue.