ここを参考に

$ rpm –import http://packages.atrpms.net/RPM-GPG-KEY.atrpms

$ rpm -ivh http://dl.atrpms.net/all/atrpms-repo-6-7.el6.x86_64.rpm

$ rpm -Uvh http://dl.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-5.noarch.rpm

$ yum –enablerepo=atrpms install ffmpeg ffmpeg-devel