I got the same buzzing sound with my switch to PC Speakers setup too, seems like a grounding issue as others mentioned, I managed to find a cheap fix with a ground loop isolator, you should give it a try:

https://shopee.com.my/product/47653752/1457970495


P.S.:
If your Monitor and Speakers are connected to a PC that is always on(or is on when you're using the switch), you can plug your 3.5mm jack into the PC's line-in port, configure your PC to "Listen" to it and you will be able to control the volume via your PC's volume mixer.

It's pretty cool, you can even have a Switch's icon in the volume mixer. I'm not sure if this affect the sound quality but I'm not an audiophile so I can't really tell a difference. This works best for me as a one time setup and eliminates the needs to mess with cables and volume each time my Switch is docked.

This post has been edited by Kellicros: Jan 3 2019, 02:18 PM