Soudset is typically for the "voice" or "character" sub-folders in the audiobank folder. Its a bit confusing because when you click the drop down it doesn't allow you to choose a voice folder but only an audio clip. Which isn't how this field is suppose to be used for characters.
I did report this issue to the github issue tracker:
to use the soundset feature you have to just type the name of the folder of audio you would like the character to use. Which can be found in audiobank/voices/
folder. (I can't recall exactly which folder GG uses since I'm at work I cannot check.)
Some examples are:
the way soundset works is when a character uses something like the commands in LUA:
For example, it will scan the folder you have set for soundset and play the audio file associated with the call randomly. This way you can have many sounds for if an enemy is hurt instead of just one and things like that.
It's important to note that soundset is very different from the soundset# fields. As those are usually defined so you can reference with them with: PlaySound(1,e)
EDIT: There are some bugs with this. So looks like you helped discover one! I can't get any solider enemy to use a soudset. They are silent.