Due to the mostly cylindrical shape of Melomania 1, it is not always easy to ensure the microphone is facing towards your mouth. As they can be placed in any way, it is possible that the microphone could be pointing backwards or into your ear which may cause distortion.
To ensure the correct orientations, we would suggest placing a finger over the hole above the L and R letters and ensure that this is pointing forwards as you insert them into your ear and see if this helps the issue.