How to disable Microphone in XPS-15?

The microphone driver is part of the overall audio driver in the system, not a separate driver.  If you uninstall that driver, you'll be rolling back the entire audio device to the default Windows drivers, which isn't recommended.

Also, where are you choosing to disable the mic?  It's not listed in Device Manager as a separate entity.  However, I just tested on my own XPS 15 (9530 generation, not sure which one you have) and when I go to Control Panel > Sound > Recording tab, I can right-click the "Microphone (Realtek High Definition Audio)" and select "Disable".  At that point, when I try to use Skype For Business to join a conference call, I get an error that I need to set up an audio device, so it obviously noticed that I disabled my mic and didn't try to re-enable it. This isn't ideal because at least with Skype For Business, I can no longer even JOIN a conference call to listen while muted, but that's probably just the way this particular application is configured. One alternative rather than disabling the mic entirely in that Sound interface would be to simply double-click it, go to the Levels tab, and mute it, although it's possible other applications will change that setting in the background.

In any case, muting one mic does NOT automatically mute other devices; volume levels and mute settings are per-device and always have been.  That said, most applications can only be configured to listen to one audio device at any given time, so if they're configured to use your external mic, then they will ignore any audio heard by your internal mic anyway.

It was a series of things that I did. First, I use the “Sound” control panel to disable the microphone.  Then I went to an online microphone test site and found that the test site was still able to hear sounds from my mic.

Next I used the computer Device management UI to Disable the device.  After disabling, it no longer appears in the Sound control panel.  Even when I “Show Disabled Devices” the built-in mic no longer shows up…but…it works fine when I go to an online audio test site.

Next, I used the device manager to uninstall the built in mic.  Now it no longer appears in the Device manager UI…but it works fine.

You are the ONLY person who has told me that the mic can be disabled.  Everyone else I described this situation to who then tried the steps above found the same as I did…there seems to be no way to be sure the XPS-15 microphone is not picking up sounds.

Can you link to the site that successfully picked up sounds while your mic was disabled?  I could try it on my XPS 15 while it's disabled in the manner I described and let you know the results. If it picked up sounds while your mic was disabled, it sounds like it's deliberately NOT respecting your Windows settings but rather trying to get around them.  Is that the scenario you're trying to address?  I thought you just wanted to make sure that you weren't inadvertently heard while you were on a conference call and thought you were muted, for example, in which case regular disable mechanisms would work fine.  But if your concern is more along the lines of malware, then any of these disabling mechanisms are software-based, which means that other software (such as malware) could potentially reverse those changes in order to re-enable the microphone if they gained enough access to your PC.  If that's what you're trying to address, then yes you'd have to physically disconnect the mic, which would probably involve disconnecting the entire webcam since I believe the camera and mic share a connector to the motherboard.  Speaking of your webcam, while you're at it you should probably also place a piece of opaque tape over your webcam, since occasions of remote webcam activation and video recording by the NSA and even a school district in Philadelphia has already been known to have happened and has been widely publicized.

There may be an intermediate step if the BIOS setup allows you to disable the mic or webcam (which might include the mic), so if you haven't checked for an option yet, you may want to do so.  That would of course be more convenient than opening your system and be a "lower level" mechanism for disabling that hardware, but technically even THAT could be reconfigured given a sufficiently knowledgeable piece of malware with enough access to your PC.  You have to decide where you want to draw the line between assurances and convenience.

I was mistaken about the last step.  I guess I got interrupted before I finished trying things.  

When I uninstall the mic from the device manager then it does what I want.