First please make sure that the connectiong cord of the MMX 300 is plugged in completely into the housing of the headphones.
Simply pull out the connecting plug and press firmly into the socket.
Other problems could be:
1) Software settings
a) Soundcard settings
b) Windows settings
2) Hardware defect on the sound card, damaged microphone jack
3) Hardware defect on the microphone
If this doesn't help please check if other headsets / microphones are working with the soundcard or not.
As a last test you can try to connect the MMX300 with a mobile device (CTIA pin assignment such as a iPhone) to see if the microphone works here.
Of course the microphone of the MMX 300 could be also defect, however the error rate is very low here.
Software settings are the reason why it doesn't work in most cases.
If none of the tips work please contact our local service department