You can go to the unit in the Data module and click on its associated sounds, and then open up the 'Sound Assets - File' for the sound by double clicking that field, and then remove the listed sound files by right clicking on them and 'Remove Sound'. I think that should remove the sound preview that is constantly played in the (terrain?) editor.
**Note: This will also remove the sounds from those units in-game.
I can't find the "bubbling" sound. I can find Infested Colonist "Yes" "Select" and that kind of voices but not the one that causes sound in editor and ingame. I belive this has something to do with "sound emitter". How to remove sound emitter?
Go to the Data module and then open up Actors tab by hitting the small green plus sign and going to 'Edit Actor Data > Actors'.
Search for 'infested civilian'.
Select the actor 'Infested Civilian'.
On the right, double click the field for the section '(Basic) Event: Events - Send'. (The part on the right side of the division under 'Infested Civilian').
Scroll to near the bottom of the list that pops up and right click the flag that's labeled 'Actor Creation - Create InfestedCivilian_Moan' and select 'Remove Selection' (Note: Must right click the flag part, not the action part).