Hi,
Have you tried checking in Recording Devices if it's detected or not? Based from Turtlebeach Support page it won't require a splitter of any type if the PC has a headphone/mic combo.
- Right click on the speaker icon and select Recording Devices.
- If your headset is listed right click on it and select Set Default then click Apply and OK.
- Test your microphone again once done with these steps.
- If these will not work do uninstall/reinstall the sound card driver.
- In Device Manager uninstall the sound driver.
- Reboot your laptop and it would reinstall the driver automatically or download and install the sound driver from the manufacturers site.
- Once the sound driver has been installed test it again.