Exactly same, phone is fantastic but I need Skype. When in Skype and press speaker lower microphone is stopped. When press speaker again then lower microphone is on.
Nothing hardware as the lower microphone works fine in all calls or even in Skype telephone mode. As long as you not go to speakermode all is fine. When on top speaker (when normal call) no problem. I use skype telephone mode now for contacting family. Evening I use headset with microphone in cable. this is 100% software and should be fixed easy. also I tried windows 10, brr.. horrible but hoped they fixed the microphone issue, no luck and I hope, really hope that someone will create a fix or app or mini update for this. I don't understand why Microsoft doesn't fix this. Appearantly they don't care better buy iPhone where everything works and customers are take seriously