WhiteTiger:
First of all, I can't believe you can perform transcription while listening to music.
What's up? Do you use one side of your brain for listening to music and one side for transcribing? (Actually, I think we do??)
Anyway, you may get away with it since your transcribing headphones are plugged into your sound card. If you have another jack somewhere on the front of your PC or MAC, that may work. It's tough to say without knowing the configuration of your computer and how your sound device(s) are configured.
You may want to check some of these forums for more help:
5 Star Support
TechRepublic
TechSpot If not, there are quite a few public forums devoted to sound card problems, configurations, trouble-shooting, and so on. Maybe an audiophile, audio-geek (or whatever you call them these days) at one of those forums can better advise you.