Where to find songs that are not on OST

I have found a few songs in playing the game that do not appear to be on the OST / soundtrack. One of them can be heard on this youtube video at 10:20: https://www.youtube.com/watch?v=QHMEH7BFl4g

Anyone have any idea where I can find this song? I really like it and would love to listen to it.

It’s track 34, Skylark.

Ah, I don’t know how I missed it! Thank you so much!