Hi vansya! Don't worry, it could be an issue with the app's privacy or security settings. Without external destrution. The probability that your mic won't work is very low! First things first, Could you please let us know which app you used for recording your voice? Is it possible for others to hear you while you are speaking on the phone? Please let us know more details of the issue, thks.
Hi, Whitch "screen change" repair guide are you following? “almost immediately the phone lit up” -> I think it'a shot-circuting. You shoud warp electrical tape around the battery connector to prevent it from accidentally contacting the socket again. "the phone was only with power for approximately 45 seconds"->Is this happend after the screen was replaced? Maybe you should think about change the battery.