Can you use a standalone microphone for XBOX One?

No sorry not at the moment. That would only work if you were using like a gaming pc but not through console itself.

Not sure why you're going out of your way for your son's 11 year old friend and accepting that pickiness but a simple answer for you is no, the Razer Seiren X Microphone can not be used with Xbox One, only PC. To my knowledge you can't use a standalone
Microphone on Xbox One, only Headsets. Contrary to what others have commented the USB on the Xbox one is not strictly for charging Xbox One Controllers either, it is a power source and not an audio source like on a PC. You can plug in things that take USB
power such as yes charging a controller but for example I have my external Hard Drive plugged into there, an RGB headset and it powers my Astro Mixamp. Either get your son's friend a Headset with a good Mic (Logitech makes one with a B!UE Microphone) or consider
switching to PC gaming if the USB Mic is that important.

Unfortunately I do not believe there are any microphones meant to be made standalone, with just the mic piece.

But don't let that discourage you, I'm sure if you got creative you could cannabalize a normal microphone and tape it on your headphones or something like that.

If you're having trouble hearing your voice in recorded game footage, it's likely that you need to use an external microphone to capture your audio. The Xbox Series S controller has a built-in microphone, but it may not be sensitive enough to pick up your voice clearly during gameplay.

To record your voice, you can use a headset with a built-in microphone or a standalone microphone. A popular choice for gaming content creators is the Blue Yeti microphone, which offers high-quality audio recording and a variety of settings for different recording scenarios.

Once you have a microphone set up, you can adjust the audio settings in the Xbox Series S settings menu to ensure that your voice is being recorded. Make sure that the "Capture party audio" and "Capture microphone audio" settings are both turned on.

If you're still having trouble hearing your voice in recorded game footage after trying these steps, it's possible that there may be a technical issue with your console or recording software. In that case, you may want to reach out to Xbox or the software provider for further assistance.

The headset chat mixer has nothing to do with the Mixer streaming platform. It is a standalone app which is used to higher or lower the volume of your headset, and also to higher or lower the microphone and voice of your friends.