I have a B450 Tomahawk motherboard. Up until recently I have been using a headphone that used a USB connection only and it worked fine. However when I switched over to a headphone that needed a USB and 2 3.5mm ports, the computer would refuse to recognize the device. Whenever I try to troubleshoot, the PC would just tell me that my device is not plugged in. I have already installed the necessary drivers from the MSI support page but no luck.
Which 2 3.5mm ports are you using? Did you try both back panel ports and front panel ports? Is the case front panel audio connector attached to the motherboard header? Is motherboard audio disabled in BIOS (it might have been disabled since you were using USB audio)?
It might be helpful to know the make/model of your headphone(s) you started with and current.