Dell Inspiron 7591 2-in-1 Ubuntu LTS 18.04

My wife had to get a 2-in-1, and the only one the university budget would permit was a Dell Inspiron 7591 2-in-1 .  This unit is certified for Ubuntu: 

The Dell Inspiron 7591 laptop with the components described below has been awarded the status of certified pre-install for Ubuntu.
Please note that for pre-installed systems:

The system is available in some regions with a special image of Ubuntu pre-installed by the manufacturer. It takes advantage of the hardware features for this system and may include additional software. You should check when buying the system whether this is an option.
Standard images of Ubuntu may not work at all on the system or may not work well, though Canonical and computer manufacturers will try to certify the system with future standard releases of Ubuntu.

Ubuntu 18.04 LTS 64 Bit

Pre-installed by manufacturer
Testing details

This system was tested with 18.04 LTS, running the 5.0.0-1016-oem-osp1 kernel.

End certification excerpt.

Unfortunately, the sales channel available to her only supplied the MS Win version.  I installed Ubuntu 18.04 LTS 64 Bit from the standard Ubuntu distro, and with a bit of tweaking for the speakers and the use of the Nvidia proprietary (but supported) GPU video driver, everything worked *EXCEPT* for the internal microphone.  An external USB microphone does work.  How can one get the microphone driver/configuration?  She will arrange to license the Dell port of Ubuntu as above if we can get.  A call to Dell Tech Support using the Service Tag only had the end support person (not the dispatcher) hang up on me as the Service Tag is for MS Win not Ubuntu.  If she had  been allowed to get the HP equivalent that I suggested, I have had no problems in the past downloading proprietary HP ported drivers and installing these from HP for a HP supported Linux distro (typically. Enterprise Linux not Ubuntu).  Any help would be most appreciated.