I just downloaded the FLAC of the soundtrack and it's missing track 5... and I can't seem to figure out how to send a message to GOG support about it without starting chat.....

People have posted some workarounds for the support bot. (It should let you create a ticket eventually once you exhaust its options, but it's obvious a bot can't resolve this so it would only waste your time.)

https://www.gog.com/forum/general/can_i_somehow_use_the_old_form_for_troubleshooting/post2
https://www.gog.com/forum/general/adalia_fundamentals_fixing_gog_so_you_dont_have_to/post3551