"BGM" means BackGround Music, so it may not include "bgm" in the name, perhaps I should contact the devs
It's a royalty free song named "The Flower of the Wilderness" by Wingless Seraph. I'm not going to post a link here in case it's against the rules, but Wingless Seraph has a website where you can download all their music for free.
(You can't legally reupload it to youtube without buying a liscense from them though)