In fact, the sound coming from the Wii is normal WHEN my Xbox is also turned on !
I’m planning on using my Extron System 4 LDxi which is a switcher / line doubler. The audio ports are Phoenix type (I think it’s what they’re called) and I could buy adapters here : https://www.retrogamingcables.co.uk/audio-video-accessories/STEREO-SOUND-RCA-PHONO-TO-PHOENIX-5-WAY-ADAPTER-FOR-EXTRON-MATRIX-300-SWITCH
But it would cost me £15 including shipping when a Y splitter/combiner could do the same for less than a fifth of the price. I’m waiting for BNC to RCA adapters to arrive, I’m not even sure yet I’m going to use the Extron LDxi. The last time I used it, I was a bit disappointed with the results (but back then I was hoping to use it as a line doubler but now I’m just planning on using it as a simple switcher).
“If you want to ensure isolation you need to use a 2 (or more) in to 1 out switch.” So a TRRS would do the trick ?