I have a 2.1 speaker system. It has a mini jack cable that plugs into the computer, and the two red and white RCA inputs, and a mini jack input.
I have my PS3 connected to the red and white RCA inputs, and don't have any problems with that. While that is connected, I can have the PC connected with the 1-way mini jack cable without any problems with the audio. The problem is, when I connect the RCA cables from the cable box into a RCA to mini jack adapter and plug that into the mini jack input, there is a buzz or humming sound. I tried unplugging the PS3 audio cables and plugging in the cable box RCA's directly, but the same problem happened, so it isn't a problem with the RCA to mini jack Y adapter. If I disconnect the PC audio while the cable box is connected via either directly or through the adapter, then the noise problem stops. Same thing if I disconnect the cable box audio while the PC audio is connected.
So there is a problem for some reason with having the PC and the cable box audio connected at the same time. The RCA cables are new.