The trailer uses the arrangement of the FE main theme from Fire Emblem Origins.
Neither of the devs, Helious and @boye have said anything about anything from that project being F2U. Just because a hack appears to have been abandoned does not give one permission to salvage assets from it. I think that FE Origins had a great OST (the OP on the thread specifically points out the sound room) but you should ask permission before using their music according to this sites rules.
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