For some reasons, the authors and publishers of the Mysterious Cities of Gold season 1 (1982) had the brilliant idea to never release the entire soundtrack ever, leaving us with only a vinyl of a few songs (https://www.citesdor.com/musiques/fr_musiques_vinyle.html). Thankfully, some fans of the series have taken chunks of the missing songs by ripping them from each episode then sticking them back together, but since they are from the show and often contains noises and talking, the quality is terrible (they did a great job, though, (https://www.citesdor.com/musiques/fr_musiques_reco.html) and the following is probably the hardest of all MCOG tracks to find (http://www.tzone.org/~pat/anime/mco/tao.mp3)).
Some people, however, have decided to re-orchestrate the entire soundtrack from scratch and even made remixes or new songs using the original instruments and synths. You should check them out at some point:
Newer re-orchestrations and new songs: http://le-grand-heritage.com/musiques.php
Older re-orchestrations: https://www.citesdor.com/musiques/musiques_boub.php3 (misses several tracks, but some versions sounds nicer there)
I hope that the soundtrack for season 2 and 3 comes out at one point (although season 2 didn't have as many memorable tracks, some were quite true to season 1)