I bought this same model last month, and I'm having the same microphone issues--intermittently.  Dell wants me to follow a multi-step audio BIOS/diagnostics/driver process that feels like a hammer.  I'm resisting until I rule out simpler possibilities. Update-" I think I might have found the issue: I recently turned on "original sound" in Zoom, which in hindsight was when I started hearing complaints about my low sound from people in Zoom meetings. "
I see on here that this might not be an isolated issue.  I was afraid that since my brand new laptop had to be repaired earlier this week to correct a loose touchpad issue that the technician may have done something to my microphone.  I "don't think" that is the case though.  The microphone on this 9500 is very weak...I have tried both the Dell Audio Drivers with MAXXAudioPro as well as generic UAD drivers from MS Update Catalog.  Sound is fine but microphone is weak.  I have looked at the MAXXAudio options and nothing really changes...especially using MS Voice Recorder as my test software.  I noticed that, unlike my older Dell XPS 13 9350 (which sounds fine), there is no "microphone boost" in Windows 10 for this laptop.  Please help.  Please confirm if you've noticed this problem as well and if you found a fix, or whether it might be application specific...i.e. it will work if/when I need to actually use it...Skype, MS Teams, etc.
Dell XPS 15 9500 microphone