mpv nuked the AUTHORS file in their upstream so the build fails because AUTHORS missing. https://github.com/mpv-player/mpv/commit/9488d312c11ab1b4267b33eb85adcb28b25a45ba Please update the ebuild accordingly.
# ChangeLog for media-video/mpv # Copyright 1999-2013 Gentoo Foundation; Distributed under the GPL v2 # $Header: /var/cvsroot/gentoo-x86/media-video/mpv/ChangeLog,v 1.28 2013/09/30 17:48:24 scarabeus Exp $ 30 Sep 2013; Tomáš Chvátal <scarabeus@gentoo.org> mpv-9999.ebuild: Drop AUTHORS file, no longer distributed.