Why can I not hear any sound? It says it is connected.
I even turned off power to my logiteck sound system.
I could use them as a very comfortable ear muffs.
What do you think ?
Thanks.
I have the same issue with my usb-c earphones that I use mainly for my phone - the solution I found was in the sound setting:
Open Sound Settings... ---> Go to "Output" ---> there on the lower part of the window should be an option "Connector:" and there open the choice menu and see if it pops up "headphones" besides the default speakers option
Because if it connects it just might not switch the output automatically for some reason idk why but that helped solve my issue and hope it helps you as well 
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Your idea worked for a while, but then the headphones shut off. The solid blue light went dead.
In the future I will see if something I want to buy is on Linux's approved list of compatible devices.
It looks like hardware manufactures concentrate they efforts on improving Windows hardware.
Does the US military use Linux or Windows?
The switch saved a further US$15 million in datacenter operating costs. By 2007, the United States Department of Defense uses Linux - "the U.S. Army is the single largest installed base for Red Hat Linux" and the US Navy nuclear submarine fleet runs on Linux, including their sonar systems.
Here are some screenshots showing what I have done.
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If it was working for a while, then the light went off - did the headphones run out of battery?
No, the battery was fully charged.
I am glad it at least worked for a while since now you know its not the Ubuntu MATE problem but the headphones compatibility problem 
And also a question on the turning off part: when they shut off did they do it in the middle of a sound (movie/song playing) or was it in the moment when there wasn't any output going on for some time? Since it might be they went off to preserve battery (a wild guess on my part but worth to check out just to cross it off the list of possible problems)
I usually go shopping for tech in my local tech store since the staff there really knows much more than I when it comes to "compatible with Linux" new tech so it saves me a problem in that department at least (I know shopping online is more convenient but it also takes me longer to browse a dozen or so forums to crosscheck specific merchandise compatibility with Ubuntu/Linux. Most of the time some stuff is in the gray area so I rather won't spend money than later be sorry I did (I mean I won't be sorry I will give it as a gift to somebody else but I won't be happy not using it and having to buy new stuff lol)
You are lucky if your local tech store knows about Linux. I usually get 'what's Linux?' when I go to a local tech store.