If it is a single jack headset, but you split it to connect to two ports, then it is likely either the port is the problem, the splitter is, or the headset mic isn't working.
I would test the headset on another device to make sure that it isn't the problem. Make sure this is a device that does not need the splitter. If it works fine, then you are down to the splitter or the port being the issue. Or perhaps the system.
While it is connected to the computer (through the splitter) try the following...
- Go to "Control Panel".
- Click on "Sound".
- Click the "Recording" tab.
- Click on the device you are trying to fix.
- Now click the "Configure" button and then click "Set up Microphone".
- Once that is complete, click the...