I'm pretty shure Triggerhappy is running, because if it was not running it wouldn't have worked for the shut down hotkey.
I found the "name" of the buttons by using the "thd --dump /dev/input/event*" command in the post you recommendet and created a .cfg:
BTN_BASE3 1 /usr/bin/amixer sset PCM,0 2dB-
BTN_BASE4 1 /usr/bin/amixer sset PCM,0 2dB+
The buttons are connected via USB to the Raspberry and work great with the commands configured in emulationstation. The Speakers are plugged in to the 3,5mm klinke output of the pi.