You have 3 options:
1)Buy a new set of speakers that supports dual audio input.
2)Use the same set of speakers that you PC has. You just have to plug the audio output from the Nintendo Switch to the audio input of your PC system. That should be on your motherboard (if you don't have a dedicated add-on sound card), or the sound-card you may be using. Just make sure that you enable the audio input in the sound mixer. However in order for this to work, your PC must be operating while you are using your console.
3)Finally you have the option of using the same set of speakers with both of them, but you have to plug, unplug the audio cable every time you want to use a different system.
As you can see the 3rd option is a little inconvenient and you are risking damaging the audio cable in the long run. However it's also the cheapest option and you don't have to run your PC every time you want to use your console. So you have to decide what works best for you. Good luck.