This is a very simple project, nearly anyone could build it, especially if you start with ready made cables and just cut and splice them. That way you don't have to solder directly to the jacks, which can get tricky.
I used 10,000 ohm 1/4 watt resistors, but anything near that value should work fine.
Some people may think of replacing the 4 resistors with 2 potentiometers. I reasoned that this is unnecessary because each computer has volume controls and so do the speakers. (usually)