I use Alien_Bob's SlackBuild for ffmpeg 0.11.2 and ran into the problem that it is not compatible with sox-14.4.0 in Slackware 14.0 due to changes in the ffmpeg API.
As it is annoying when a Swiss Army knife has a blade that will not open, I investigated further. Using patches for Gentoo and Arch posted at http://sources.gentoo.org/cgi-bin/vi....1&view=markup
together with some alterations to suit the new avformat_open_input API call I have successfully recompiled sox with support for ffmpeg on Slackware 32bit and 64 bit systems.
On the off chance that anybody has an interest, I am attaching a modified SlackBuild for sox-14.4.0 together with the patch file I have created (remove the .txt extensions). To use these, download the source for sox-14.4.0 and add a directory ./patch. Copy the patch file into this directory. Rebuild sox using the modified SlackBuild so that the patch file is used. I used 'upgradepkg --reinstall' to install the newly created package.
Now 'play <somefile>.m4a' should work. This might be useful to someone with an iTunes music library.