If you're getting a good picture, I doubt the issue's cause by a bad splitter. How was the TV's audio connected to the cable box? Was it HDMI? If not, check the audio output settings on your cable box to be sure it sends the audio to the HDMI output. Other than that, it sounds like you have everything connected properly, so one thing I can think of is to check the grounding of the coax where it enters the building. There should be a nice, tight, solid earth ground going from the junction to a rod sunk into the ground or otherwise connected to a solid earth ground. Also, what are your components' AC power cords connected to? Do you have good quality line-conditioning/ isolating power distribution...a UPS connected to your PC? Even the UPS outlets not connected to the battery backup will have AC line conditioning/ stabilization. Plug your cable box in there. You can't have any surround at all, from an analog audio signal with a stereo headphone jack, but you most likely already know that. Does your cable box have a TOSLINK optical out and your PC an input (SPDIF) for the same, to experiment? The HDMI and TOSLINK are both digital audio, BUT the headphone jack is analog only. Usually, you only have hum with analog connections, so I think you're on the right track to suspect the coax/ cable box. You can even try the analog audio outs (no surround signal) from the cable box, to the PC, but you may need to go to the box's audio setup and choose the audio out. Clean the headphone jack by spraying electrical contact cleaner into the jack, then quickly plug and unplug the plug into the jack several times. Hum can also come from a dirty/ unused jack and since this is a different monitor (new? used?), don't ignore the jack. Start with the jack and the audio out setting on your cable box. Hopefully, one of these ideas will work. Good luck! If none of this works, post your monitor, cable box and PC/ sound card model #'s.