KaraFun does not enable you to hear your mic via the app. In order to do that, you will need a mixer or dedicated software.
Any microphone that you connect to your device and use with KaraFun will be controlled entirely by your device. KaraFun only controls the volume of the vocals and the general volume of songs.
If you are having trouble hearing your voice through the microphone, try checking your device's sound settings to ensure that the microphone is recognized.