I would suggest: We can't hear you. Get closer to the mic or turn up the volume.

Your sentence is close, but it could be slightly clearer. Here's a more natural way to say it: "Sorry, I can't hear you. Your microphone isn't picking up your voice well because it's too far from your mouth. Or more simply: "Sorry, I can't hear you well; your microphone is too far from your mouth."

Sure. You could also add "your voice" as in "picking up your voice" and then you could also add something like, "Perhaps try moving (or holding) the microphone closer to you". If the microphone is on a stand, then it should be "Perhaps move closer to the microphone." That should get the message across clearly without sounding rude. I hope that is helpful.

You can say that, but it's better to give "picking up" an object: "Your microphone is not picking up your voice" "Your voice is not being picked up"

Q. If I can’t hear somebody because they are holding the microphone too far away from their mouth, …., A. Sorry. I can’t hear you. /Hold the microphone closer. /You’re holding the microphone too far away. I wouldn’t use ‘picking up’ if you know the reason that you can’t hear them. It’s pointless.