I got the .fsb files from the bgm folder in (abdata/sound/data/bgm) owning ai-girl though using SB3U utility (you'll find it in the modding tool folders). That .fsb is an audioformat for games or whatever so you need an FSBExtractor (got a version
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but the extracted .wav is written to "C:<username>/AppData/local/Temp/FSB Extractor/" which is really annoying, idk where to change the output dir lol, but at least it works. The tool is old but still works on win 10 here.
Edit: To
extract export the audio (.fsb files) load the unity3d file in SB3U
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