Sounds like your computer is outputing a lower voltage level than the rest of the equipment is looking for. I don't know what voltage levels are normal for car audio, but home audio tends to run with a peak output voltage in the 1.2V area, but sound cards often don't go that high, some go to 1.0V, some even less.

For home equipment the solution is to use a pre-amp that will let you boost the voltage. I'm sure something like that exists for car audio, but I don't know enough about it to name one.