I realize this post is old but I ran in to this same issue on a much newer verison of recalbox and this thread was first when searching "how to get sound to work on recalbox". I thought I'd post what I did to fix my issue to help other fellows.
I have speakers in my monitor that Raspberry Pi only recognizes when the connected monitor is turned on BEFORE the Pi is turned on. Once this is done, go to main menu > sound settings, and make sure your settings are to your preferences. Note: I don't use auto for the output device (the auto setting has created problems for other users in the past).
So if hardware is not working at all with your Raspberry Pi, trying connecting it to the Pi, turn on said hardward, then turn on your Pi.