Here are my specs,Update your post to include full system hardware specs and OS.
Make and model headphones? Do they work on other computers or audio devices. Do other headphones work on your computer?
Determine if the problem follows the headphones or stays with your computer.
Windows 10?
Right click the small speaker icon usually located in the lower right screen corner.
A menu should appear that will lead you to numerous audio related Windows, tabs, and properties.
Explore first without changing anything. Then try making changes but change only one thing at a time in case you need to go back an undo a change.
What audio devices do you see listed in Device Manager?
Try the built-in Windows 10 troubleshooters they may find and fix something.
I want to be able to connect the sound from my Numark NV controller into the computer."DJ Equipment" - what all are you trying to connect and what is the overall objective.
Just audio, video clips voiced over, combining tracks, etc..
Moving thread to Home Audio & Theater.
Im trying to find a soundcard that has RCA inputs, with 2 ins and 2 outs.Yes - there are a variety of cables/adapters available.
Your requirement is likely a stereo RCA to 3.5 mm (male) plug.
For example:
https://www.amazon.com/AmazonBasics-3-5mm-2-Male-Adapter-Stereo/dp/B01D5H8LDM
The Numark's RCA outputs (red and white) would be plugged into the audio in port (3.5 mm female) on your sound card.
Note that the link is indeed an example. You must take a close look at the plugs, connectors, sizes, and genders on both ends.
Here is a link to help you get started:
https://www.majorcom.fr/web/en/569-le-cablage-electronique.php
As you work through the details, revise your search criteria accordingly.