This has helped me. May be it might solve your issue too.

Uninstall all realtek drivers. Let system detect the audio drivers. Then follow below steps.
Enable Waves MaxxAudio at Startup:

• Go to Task Manager > Startup or Settings > Apps > Startup and enable Waves MaxxAudio Service/Application.
• Restart your system and plug in the headset to trigger detection.

Do you have the Dell Optimizer installed?

I had problems with the MIC on my headset until I installed the Dell Optimizer perhaps that'll fix your issue too.

I installed it yes.

I will try to uninstall but I think I had this issue before Dell Optimizer.

Thanks for the tips.

This is Dell's updated article on audio issues:

https://www.dell.com/support/kbdoc/en-us/000179430/how-to-troubleshoot-audio-or-sound-recording-issues

The problem you have only occurs when you are docked? 

What external audio system are you using? Is it a USB headset or something else?

What dock are you using?

Unfortunately uninstalling Dell Optimizer didn't solved the issue

The problem occurs even if I'm docked or not.

If I plug the headset in the jack, it's not detected.

I already reinstalled the audio drivers many times with different versions to be sure.

Luckly I have a USB headset so it's working this way but not by the jack

 

Even on the jack with the docks (WD15 and D1000 - firmwares up to date) it's working sometimes.

 

There is 2 USB-C ports for this laptop.

I need to test again but depending on the USB-C port choosen it's maybe better.

I have a Dell 34" curved monitor and the sound is not travelling to the audio system.

It was working fine with my Dell 7490 before.

OK, so is the monitor the P3421W? Curious.

Is the monitor connected to the dock, and the laptop to the dock?

When you say the microphone isn't always detected, are you indicating with the input on the dock, or on the laptop, or both?

The 7420 has a combination headphone/microphone combined or "universal" port. It needs a special cable if you wanted to just plug in a special microphone, or you wanted to use a headset with separate microphone and headphone plugs.

On the WD15, on the front (with the fewest ports) there is also a combo microphone / headphone jack. On the back (with the most ports) there is a "line-out" jack for use with an external amplified set of speakers.

Normally, I would say, if you were using a headset/microphone with a universal port, the "line out" that you might use with say, external amplified speakers wouldn't work, unless you used the Sound Device control in Windows10 to indicate where you wanted the audio out to go.

Also with the sound control, in Win10, I would think you could select between the microphone that is built-in to the laptop or the one on your headset/microphone (or external microphone).

In otherwords, is your problem that you haven't experimented with the sound control in the lower-right corner of the taskbar to select the input and outputs you want to use?

My monitor is the U3419W.

Now I'm using the USB-C cable of the monitor to plug the laptop.

But same issue that I have with the 2 other dockings.

For the jack I though about the combo issue too but there is 3 rings on it so it should support this function.

I don't understand.

And i have an issue with my external speakers which are not always detected.

When it's working I'm able to choose the audio device I want from the sound icon in the tray bar of Windows.

I plugged a 2.1 kit on the monitor through the internal jack and normally using USB-C it should propagate the sound from the laptop to the external audio kit.

It's working well with my Surface Book 2.

I'm totally lost with this issue.

The sound from the laptop speakers is so bad