I have deluxe edition bought. I see the download! folder in Stalker2 folder. There are installers for deluxe content, ost and artbook and pre order bonus. I executed those installers. I understand that deluxe and pre order bonus are found ingame trough quests. But, where is the digital soundtrack files?

The OST is the the digial soundtrack - just extract the the songs from the archive file? You don't instal the ost - its usually an archive

Right, at start I did not see the OST mp3 or flack option at the GOG galaxy, just backup installs. Now it is.