Look into the music from create astral. Ask them/check license

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yarden.zamir on Discord says
They have awesome music for the pack which should fit really well, need to ask them about using it in our pack

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yarden.zamir on Discord says
I'll say pending/contributors/research contributors/research would be a good fit for this. Anyone up to pick this up?

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auseawesome on Discord says
Its ARR so we'd have to ask for permission specifically
The official OST seems to have been published separately as well, so it's all custom music
Your best bet if you like it is probably reaching out to them yourselves and then maybe even offering them a cut of money from the modpack