One of the various story BGMs in D4DJ Groovy Mix (or グルミク / Gurumiku for short).
Though this is technically from one of the D4DJ Animes (but is not in the official First Mix OST release, from what I can tell).
This track first plays in the new event story 「side：origin -Lyrical Lily-」
Titled this video “DS Anime 51A” based on its file name in the game (bgm_adv_anime_DS51A).
Playlist of the game’s other BGMs: https://youtube.com/playlist?list=PLgeF9YHImSactvdda_3g2UjxP89uHSosF
Official streaming links for the First Mix OST: https://lnk.to/d4dj1st_osttw | https://www.youtube.com/playlist?list=OLAK5uy_npz_74HIIaKDRm9rTdOJtP1fVMXTTUhA0
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In which folder is the music file located?