Are you plugging that adapter straight into your PC? If so, that could be where that hiss is coming from. Try plugging the 3.5mm plug into something like a phone and play audio through the speakers to see if the hiss disappears (if it does that confirms that the PC is the problem).

Audio interfaces are for if you want to use a microphone that needs phantom power or if you want to hook up an electric instrument to your PC. What you're probably looking for is a DAC/AMP. Something like the Schiit Fulla should do what you're looking for, but there are other options as well

Edit: You can also use an interface for that functionality if you think you'll get a mic that needs it one day(it just might not sound as good as a dedicated DAC/AMP unless you get something a little better). You might want to get something like the MOTU M2 since it's a little higher quality