After that is selected click 'Okay' and close the previous sound window. Most likely it will be your 'Default Playback Device' but depending on your setup you may want that to play to just the output the headphones are connected to. Click it, then click 'Properties' then on the 'Listen' tab there is a checkbox for 'Listen to this device' select that and choose where you want to have the sound played to. Select the input device, most likely in this case it will be 'Microphone' with a subtext that has the name of your soundcard. With that open click the 'Recording' tab. In the control panel in all the Windows versions there should be a 'sound' option. If that's not the case there should be a similar setting in any other operating system it just won't be found as described in the following instructions. Since you tagged this with PC I'm assuming windows of some variety from XP to Windows 10. You can get to these settings several different ways and it may vary depending on the operating system. One for the headphones and one for the microphones. You have a stereo to stereo (TRS to TRS) splitter and not a stereo to 2 mono or Left vs Right splitter. The most likely reason this stopped working is a setting change. Yes this will work, it's certainly not designed to function this way but it will work.
