A long, long time ago (2002), I wrote a little utility that used xosd and ALSA events to show volume bars whenever the volume changed (I was trying to make my desktop pretty). See the screenshot for a better idea:
As the screenshot makes obvious, I wrote this in the days of Netscape. I have no idea if it works today. This was also designed purely for personal use on my machine. It may not work anywhere else.
I threw up the source code on my web page, with no project web page, and essentially no documentation. I then forgot all about it, and the code began to bit rot. To my great surprise, someone found it, and used it. Indeed, when the xosd API changed, Marek Szuba of CERN sent a patch (2008) to make aosd work again. In response to the patch, I created a web page for it.
Shortly thereafter, Marek generously agreed to continue maintaining aosd, so future updates were be on his aosd web page on SourceForge. I rather prefer when things on the Internet don't disappear, so I kept the page up for historical reasons. In 2016, Sourceforge was on the way out, and a tarball on my personal page seemed quaint, so I placed a copy on github. The SourceForge page is canonical, though, so long as it is still up. Marek is the maintainer.
gcc aosd.c -lxosd -lasound -oaosd
I have not tested Marek's patch, since I no longer use aosd myself, but I did read it to confirm that it contains no hostile code.
The code is distributed under the GNU General Public License, and comes with no warranty.
Copyright (c) 2003-2016. Piotr Mitros.