Need help with finding a compatible soundcard

I just bought some new speakers, they work with the AUX output which is 2.1, plugged into my monitor. I do know that my speakers are 5.1, if I have the right soundcard. Which im willing to buy, I just simply don’t know what to buy, or if it will work for my PC or not. Can anyone help me?

Motherboard: Z170-P ASUS
Budget: 100$ (Flexible)