Transmission line effects at audio frequencies on even relatively crappy cable won't be evident at 100ft. If you use halfway decent cable, you should be fine. Be aware that most PC sound cards don't put out a great signal in the first place, though, so if you have problems with noise levels, you may want to put a small preamp to boost it up to "pro quality" line levels (which are generally much, much higher than that found on consumer gear) before running it through your cable run. Most likely, the only factor on a run of that length will be attenuation.
If you still have noise or other problems, you can move to a balanced line, but then you'll need balun converters, and you probably don't want to bother. You could also move to TOSLINK or other forms of optical audio transmission as they don't exhibit transmission line effects except on unusually long links with poor quality cabling.