Soooo, by a process of elimination, it's not the headphones, and as you've tried both front and rear sockets on the PC, it's not either one of those, so assuming they are both connected to the same sound card, it could be that (just try reseating it at first, it might just have some dust in the contacts), or possibly, does the speaker system on the PC have an external volume control? Try wiggling that a bit, and see if it crackles. Sometimes that's enough to dislodge any dust which might be in the switch.