Mic only works for a few seconds

So I have kind of a weird problem with my mom's iPhone SE, the mic works but only for a few seconds.

The microphone works fine for phone calls and speaker phone but when I try to use dictation or try to activate "hey Siri" it only picks up a few words and then cuts off. I thought it might be a hardware problem because we bought the phone used so I tried replacing the mic assembly, still not working properly.

I believe this is a software problem but I'm not sure how to fix it and it doesn't seem to be a problem other people have had.

Any help is much appreciated.