The soundtrack zip file ("julia_ats_ost_mp3.zip") and the game's DLC/Soundtrack directory both contain file "JULIA_AtS_Soundtrack_list.jpg", which gives a listing of the individual tracks. According to that listing, there are 20 tracks, but the soundtrack (zip file and DLC directory) contains only 19 tracks. Track "Piknik - Credits" (#19, according to the listing) is missing, and track "Whoop" is #20 in the listing and #19 in the soundtrack. Interestingly, the bottom of the listing says "Composed and performed by Jan Kavan except Piknik by Metamorphosis".

Thank you for posting this.
Years later I find this and can safely remove from wishlist for 2 reasons. Incomplete OST and ignoring gog forums.

The soundtrack on Steam is also missing the Piknik track according to SteamDB. Here's the listing on Steam: https://steamdb.info/depot/257701

However, on Steam, the soundtrack + hintbook + Untold game are in a separate, paid DLC. On GOG, they are included with the main game, strangely.