How do I control Macbook volume when a DP display is connected? When I plug my Mac  into a DisplayPort monitor that has loudspeakers, the Mac's volume controls become disabled.  

Is Apple assuming that if I have external speakers I am so much better using their own volume control that I have to be forced to do it?   That is annoying because the monitor's volume controls are buried in terrible menus accessed by terrible buttons on the back of the monitor.

Also, if I am inside an app where I can select the audio device and I select the Mac's internal speakers, the volume controls are STILL disabled.  Is there no way to control the volume of an app when an external speaker is connected even if the app is not using the external speaker?????

Some additional info: It's a Macbook Pro 13" 2018 version.  Using a Thunderbolt to Displayport cable, plugging in to a Dell monitor with speakers.    In another location I use the same cable to plug in to Dell monitors with Displayport but without built in speakers in the monitors.   At that location the volume controls remain active and they control the internal speaker in the laptop as they should.
MonitorControl Control your external monitor brightness &amp; volume on your Mac. It does not require installing a driver.
SoundFlower MacOS system extension that allows applications to pass audio to other applications.