Sum up: Output Switch audio to a destop, then output both desktop and Switch audio with the desktop. So that you can play Switch along with Discord or other chat app in the desktop.
Requirement
desktop with Win10 x1
Switch x1
3.5mm male-to-male cable x1
Step
Plug the 3.5mm cable to Switch and rear audio input jack of the desktop.
Right click the Speakers icon on the taskbar and select Sound (音效).
In Sound window, select Recording (錄製) tab.
In Recording tab, select Line in at rear panel (Blue) in the panel and check Properties (內容).
In Line in at rear panel (Blue) window, select Listen (接聽) tab and check Listen to this device (聆聽此裝置) then click OK (確定).
From now on there should be sound of Switch output from your desktop.
Trouble shooting
There’s buzz from Switch through the 3.5mm cable to the desktop
Solution1: Prepare an USB audio adapter, plug it to the Switch dock and plug the cable in switch side to the adapter, then play Switch in docked mode.
Solution2: Prepare a Ground Loop Noise isolator, place it between Switch and the desktop.