a bunch of needed files have been either left out or missed or were dropped with an update or something, cause you can get campaign and SP maps to function again, but MP maps seem to be a whole lotta trouble as there seems to be more than one missing file and/or desc.

If you want to try and fix it yourself, start the game, go to the directory listed in the error [\steamapps\workshop\content\400750\2829617084\Resource\Sound] and then explore in there until you find either the file or a file similar to the sound file thats missing. just keep looking, its probably there somewhere.. once you have found a file, create a new folder exactly where the error says the file is missing from, then paste the sound file there (after copying it). Make sure you create it exactly where the error says its looking for the file, copy and paste any folder.set file along in the new folder you have created.

Open the folder.set file and edit the text to look like this :
{sound
{volume 1.5}
(include "\sound\defines.inc")
("PLACE_ERROR_FILENAME_HERE")
}

once you have done that, go back to the main sound directory where you will find the Defines.inc file mentioned ine text above. Once you have found the text file, edit it and add the following entry into it under the section applicable; Note: the header of the section where you are going to paste the text will define what other parameters to set. I.E: the details might change depending on which section you are placing the entry, so copy and paste from the same section and paste it at the end of the section and save. The below example is from the "HIT" section.

(define "barrel_water"
{signal hit 10}
{loudness "barrel_water"}
{tag effects}
)

(define "MISSING_FILENAME"
{signal hit 10}
{loudness "MISSING_FILENAME"}
{tag effects}
)

This is how I have gotten it to work at certain points, but as you will see, more and more sound files will keep being needed until you either give up or get them all. ALL OF THIS IS CASE SENSITIVE. BE SURE YOU TYPE IT EXACTLY AS NEEDED. If you get lost or are uncertain, literally follow the error, it tells you everything you need to know of where its looking for what. just create a what its looking for and itll shut its mouth. IDK if it needs to be exactly what its looking for, but as long as there is an entry and a file for it to find, itll be happy. That being said, im pretty sure you dont want the sound of a tank exploding whenever a soldier walks so thats the reason for trying to find a similar sound. This is not a definitive fix, you will have to make these changes each time you start the game again, my advice? unsub from mod or just wait till there is an update.. if there will be an update.