Links are broken. If this is what you have:http://www.amazon.com/Sewell-Direct-SW-22050-PC-Converter/dp/B002PXFJ2O
Then it looks like that goes from PC to TV. It's hard to tell exactly by the Q&A, given the lack of details and weak grasp on the English language, but I believe it only converts one way. I know there are methods out there to convert from RCA cables to VGA, but I have no direct experience with it. You can hunt around and look for what methods other people use to rig it together or find something that specifically states that it has VGA output and RCA input.
The sound 'splitter' should be bi-directional. meaning you can plug in RCA audio into one side and hook up the 3.5mm to an amplified device to output sound. You won't be able to simply plug headphones into it, but a set of powered desktop speakers should work fine. You could also plug it into the Line In on your computer (you need a male 3.5mm plug on that end of the splitter) and use some sort of audio manipulation program (Audacity perhaps?) which can output the sound to whatever your computer currently uses (speakers, headphones, etc) and save you from needing to swap wires around every time you try to use the N64.
There are some cheap USB based devices that can take RCA inputs and output to a screen if you have said screen attached to your computer. They usually come with software and there is also free of cost programs out there to make use of the video. I used a similar setup as a makeshift DVR a while back. If you already have 2 monitors attached to your PC then this might be the cheapest and easiest route to go. You can find them often for less than $20 and take care of audio and video. If you need the monitor to be standalone and have powered speakers for it then they make devices, similar to what people use on older projectors, with RCA input and VGA output.