If you need a specific file ( cmnrootenglish.sabs ), it’s part of the game’s official installation and cannot be legally shared separately. Verifying game files through Steam is the safest way to restore it.
A Reddit user on r/PiratedGames posted a fix for the error "sound bank failed to load cmn_root.polish" when running *Call of Duty: black ops 2 sound bank failed to load cmnrootenglish link