Actually, 100ft is not really considered all that long in many circles -- it's not uncommon to find installations that run audio AND video for this distance without any problems. People tend to assume that this length will be a problem without actually trying it, or they incorrectly blame problems on the cable length when this is not the issue. The problems that arise in these situations are typically related to ground loop and other hookup issues that have nothing to do with distance of the run.
I just did an installation where a sound card is used to send audio roughly 100ft to two different stereo systems. I called Belden and they recommended 1505A for this purpose. This is a fairly inexpensive cable that's easy to find and is quite popular for both audio (digital and analogue) and video in professional installations. Make sure that you use black if there are any exterior sections to the run since only the black is UV safe (although I would avoid direct sun anyway.)
I used 1505A terminated with Canare F-10 solder-on RCA plugs (also extremely popular in pro circles while still being very inexpensive.) This has worked well for me -- the audio is totally clean with ZERO hum or added noise resulting from this run. The only issues I had to deal with were ground loop issues related to the video -- some problems caused by the cable TV hookup, which is common. The distance, however, was not a factor in this.
It's true that low capacitance is key to long runs (guage is really not an issue.) This usually means it should be in the neighborhood of around 20 pF/ft (1505A is 16.3 pF/ft.) I'm not sure if it really gets much low than this.
Larry