Install this version:
emerge -a =app-emacs/emms-25
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =app-emacs/emms-25
Or alternatively:
emerge --autounmask-write -a =app-emacs/emms-25
# automatically generated by g-sorcery
# please do not edit this file
EAPI=8
REPO_URI="https://stable.melpa.org/packages/"
SOURCE_TYPE="tar"
REALNAME="emms"
DIGEST_SOURCES="yes"
inherit g-sorcery gs-elpa
DESCRIPTION="The Emacs Multimedia System"
HOMEPAGE="https://www.gnu.org/software/emms/"
SRC_URI="${REPO_URI}${REALNAME}-${PV}.${SUFFIX}"
SLOT="0"
KEYWORDS="~amd64 ~x86"
DEPEND="app-emacs/nadvice
app-emacs/seq"
RDEPEND="app-emacs/nadvice
app-emacs/seq"
app-emacs/nadvice app-emacs/seq
app-emacs/nadvice app-emacs/seq