Dan if the phone was consistently dropping you voice i would recommend replacing the mic but since it works intermittently I am leaning toward this being a software issue. Have you installed any apps that coincide with when the phone started to act up?If so you may want to check the settings of the apps and if it does affect the phone function . I know that some recording apps can shut off your voice . This may be the situation here. You may need to delete the apps. If this brings no joy you may need to do a factory reset to bring the phone back to original status. be forewarned that a factory reset will delete all you data, contacts, etc. If you decide to do this it can be done in one of two ways . You can go into the settings and do it from there or from the command This video shows how
https://www.youtube.com/watch?v=IdwT5fQSEGQ
Hope this helps