So this isn't about normal local wireless play, it's the one where you are all connected to the same router and you press a button combination (I think it's L+R+Left stick). It then uses the router in a LAN configuration. This is much more stable than just wirelessly interconnected Switches.
Which is why we did it last week for our Splatoon game in a pub. One of us had a microphone on his headphones, and the second we switched into the LAN lobby we heard what he was saying VERY LOUDLY in our speakers. Was quite a shock really.
This means they implemented seamless voice chat, no app required, but allowed using it only for LAN play (not even normal local wireless, WTF?) which, frankly, is probably the least useful scenario for voice chat, since you're probably all in the same room anyway.
Lock if old etc.
Hopefully someone is going to double check and maybe even properly test this before I get to try it again. If you are, here are some tips for going to the LAN lobby:
- go to the local multiplayer lobby (The Shoal)
- press and HOLD the combination of buttons: L + R + Left Stick for a few seconds, until the game fades out of the lobby. It then fades back in to the same lobby but in LAN setup
- obviously, you all need to be in the same local network, which means all of you connected to the same router (WiFi hotspot)
It's at steps 3 & 4.