Aumix is a useful mixer / volume adjuster. Using emms you can need to adjust the volume, so here it is aumix.el that allows you to do it from emacs using aumix. You can download aumix.el from here(dead link at 2012-12-4).
If you already use osd.el you may want to download aumix-osd.el to get the volume printed on screen ;) Download it from here(dead link at 2012-12-4).
To install it just put the following in your .emacs:
(load "~/.emacs.d/aumix.el") or
aumix.el and aumix-osd.el are obviously released under GPL License ;)