To fix the the mouthpiece failure, you may first check and try replace the microphone module. If the said mic is confirmed okay, you may refer to the following solution below and do have a check and test procedure on some components and connection line paths.
Nokia 2220 slide Microphone Failure Solution -Jumper ways and Tracks
You may then trace the positive and negative line paths of the microphone, do check all the corresponding components connected into it like the filter coils, capacitors and resistors. The Mic Bias voltage is needed to make the microphone works, check and read also this voltage using a multimeter. The Mic's analog audio data signal is feed to the GAZOO chip. There is also a possibility that the problem may lies on the said chip when the microphone failure occurred on the 2220s device.