To clarify, we are discussing 2 seperate issues here.
1. I understand that car stereo equipment outputs a 4v signal for low level sound (that is the audio you would use RCA cables for), while your home audio equipment, PC, headphone players, etc. use a 2v signal for reference.
Hence, you would use the earlier mentioned line driver to take the audio from your non-car device and up the voltage level (up to 13v in the three.1 device mentioned earlier) to around 4v or whatever sounds good.
2. Audio out of PC's is not the best. The PC pre-amp components are cheap, so the base level audio is notably worse to start with *before* any line drivers, etc.
To solve this some people swear by the external USB sound cards like the ones by Creative. These also have the advantage of sending a digital signal across the USB port to the device, then converting this in the device to regular analog output. This method provides at least some electrical isolation from the PC motherboard, hard drives, etc.
Others say that some on board sound cards (those built into the motherboard) sound great, or similarly to use a PCI sound card instead.
I think the 'best' solution to provide clean sound (money no object) is to get an external USB sound card that gives optical SPDIF out. That way you have some isolation up to the sound card, then total electrical isolation after it is converted. Once the signal is optical, NO noise can be introduced. Then it becomes a matter of converting that back to analog. This also circumvents the voltage issue (#1).
Unfortunately NO head unit I could find supports optical in. Some amps, crossovers, and 5.1 decoders DO however have these as inputs. Also, you can buy a device that will do nothing but change that Optical SPDIF output into analog audio.
So to summarize issue #2, if you are rich, take your PC audio and run it to an external USB sound card that supports SPDIF optical out. Then run the optical signal to another device to change it back to analog, then run it down through the rest of the system. Wala! You have completely virgin sound!
The rest of us just have to make do with the best soundcard we can find/afford, and try to clean it up with isolators, etc.