Thanks for the info. I am running the FreePBX Distro, so I should be able to do it. Did you build and install all of the encoding libraries suggested for CentOS on the ffmpeg.org wiki? Or are only some specific ones needed? Perhaps that is my problem.
Interestingly, if I install the rpmfusion.org repo, then install lame and ffmpeg with all dependencies, the notification on the FreePBX Dashboard about missing HTML5 format converters clears itself automatically after a restart. But, if I build from source, the notification does not disappear automatically. My guess is that I am not including one of the encoding libraries when I build.
Perhaps I should just install all of the encoding libraries? But then again, I don’t want to break something else that is already working…