If it has powered speakers crank the volume on the amp box and tap the 3.5mm input plug tip and second one back with skin of your finger (try to miss the far back one), it’ll make a pop and bit of a hum or buzz if working, wet your finger will make it louder again, on the off chance you have dry reptile-like digit ends.
Sound cards if recall generally had three 3.5mm sockets, green may have been speaker output, blue may have been aux, and red or pink may have been microphone input. It’s been a while, I can’t be sure, and hardly care. If the headphones work in the speaker output from sound card generally you might say the sound card is alright, but there may be exceptions, which I wont bother you with, the low care factor being the theme of the day.
If the sound card seems to have output, and the amp box tested seems to give a buzz, then you can insert the 3.5mm plug into the appropriate socket, if that is what it has, and here ends the sex education lesson and we move onto something more cerebral.