Have you ensured that you haven't 'muted' the phone?
If not then try using the earphones with the microphone attachment. Then make a call or get someone to call you.
If you still cannot be heard then there is a problem with the voice transmission circuit in the phone and it will need to be repaired by a reputable, professional phone repair service.
If you can be heard, then the voice transmission circuit of the phone is OK but there is a problem with the phone's microphone. It is either faulty, has become disconnected from the systemboard or perhaps the voice inlet hole on the bottom edge of the phone has become blocked.
To check this last possibility, use a magnifying glass and a strong light and check to see if the hole is blocked with dust or lint. If it appears to be DO NOT try to clear it using a pin or suchlike. You may damage the microphone which is directly behind the hole. Try a vacuum cleaner, using short sharp bursts to see if you can clear the obstruction.
If the hole doesn't appear to be blocked, here is a link to the Ifixit guide on how to access the microphone in your phone.
Blu Studio HD 6.0 Microphone Replacement