I tried using one of these
https://www.amazon.com/gp/product/B00DIGCZ3S
in order to be able to listen to the audio of both my PC and PS5. It works pretty well for the most part. However, it does considerably lower the PC's volume. (Curiously, the PS5 volume is unaffected, regardless of which way the devices are plugged into the splitter.) As a consequence, whenever I disconnect or turn off the PS5 the volume from the PC will suddenly drastically jump. What's more, when the PS5 notices that it's not outputting any audio—for example in the main menu when menu music is disabled—it'll also disconnect the audio; presumably to preserve controller battery. So if I'm listening to music from the PC I frequently get deafening volume spikes.
One remedy I've tried is to connect to my monitor's audio port instead. At least the PS5 doesn't toggle the signal there all the time. But then the PS5 refuses to output headset audio. So I don't get proper 3D audio which is kind of a deal breaker.
Does anybody know what's causing this drop in the PC's volume? Might using a different splitter help?