It seems Nintendo has brought something else to ver 4.0 on the Switch that did not show up in any of their changelogs. This Ver 4.0 brings USB/Bluetooth sound compatibility to what range I do not know, as I can’t find anything in their changelogs.
Unless we miss something Nintendo announced, it would do for the Switch in previous versions. It’s just like we couldn’t find anything out about this new joycon update in ver 4.0 from them. I did test the first gen PlayStation headset that comes with a USB Bluetooth dongle and using the dock or USB-C otg cable.
As soon as I plugged it in I got a notification for USB sound from either the dock or otg cable, then the PlayStation headset would connect right to the dongle stick and the sound was spot on. This is a big improvement and will hopefully be used for Multiplayer as the setup for Multiplayer right now sucks in my opinion. A game patch will probably be needed for any Multiplayer games to use this new feature but we know Nintendo is slow when it comes to stuff like this. 
Nintendo did confirm that Splatoon 2 supports native voice chat over LAN connections as-is, although all wireless multiplayer modes require players to use a separate smartphone app in order to communicate via Polygon
I didn’t test any other headsets as I only have one wireless gaming headset but other users on NeoGAF are reporting having success with using a Bluetooth dongle too. So if you have a Bluetooth dongle and a wireless headset you might want to give try it and see if your headset is compatible in dock mode or if you have a USB-C OTG cable. You can check out the NeoGaf post here.
