Mic makes me quiet

Yup. I tried with another mic and the same thing happened. Also, that mic did work before.

That sounds very frustrating, and unusual. I'm racking my brain to think what sort of odd thing could cause that, and I'm pretty much out of ideas. I know this is reaching, but do you have a Kinect? If so, can you make sure it's not able to be used for Chat?
There's a setting under Settings -> Audio that disables the Kinect microphone for chat. I'm theorizing (a big stretch) that perhaps you're not using the microphone in your headset for some reason. When you use a headset through a controller, it is supposed
to automatically switch to headset mic from the Kinect microphone, but that can behave differently if you have multiple profiles signed in or something.

The other possibility I can think of: Is there new background noise in your gaming area, like a fan or air conditioner? The headset microphones are noise cancelling, and can quiet your voice along with the background noise if the background is too loud.
When this happens, getting closer will abruptly cause your voice to be isolated better, which matches your symptoms.

I doubt either of these suggestions will be helpful, but I'm trying my best. Sorry this is happening to you.

Given what you have tried already, I suspect that this is a problem with the microphone in your headset. Have you tried with a different headset to see if that is the problem?

Hey there!

Are you using the same controller on both consoles? If you're using the same controller and you've tried both consoles and multiple mics, I'm going to recommend getting a new controller or at least trying one.

I rock a mic but just a really quiet guy (you know what they say bout the quiet ones! Haha) and yes, communication is a big help