I think there's lots of parts where there was no music. I'm guessing that's why Caribdis pointed out in the beginning of the game that "the game is better with sound/music on, especially later on" or something like that. I think that's what it said.
So, in comparison to Eternum, I'm guessing he lacked budget in the beginning - but that's one of the reasons why Eternum is a lot better in my opinion, even if OIAL is already a 12/10.
I suggest you just put on Lofi Girl, it usually fits quite well, or at least it did the trick for me. But max out the ingame volume to notice when you should turn off the music because the game has its own.