Kudos to you, bro, for making it a dedicated machine and resisting the urge to turn it into just another MAME cab. 
+10
Let's not go overboard.  He did put 5.1 in it.  
No what I did was put a good sound system in it for $80.00, and it sounds awesome, and the surround sound effects are actually noticeable.  I'm sorry you prefer 2.1 with no flexibility in range of sound or maybe even just 2 speakers connected to a kinter or lepai amp.  If I wanted baby sound I would have went that route.
So what I got with my 5.1 was way better sound, a remote for volume, so I didn't have to put a knob or extra button in the build for voulume control.  15 different decoding variations for sound adjustments.  Once again all for $80.00.  So it wasn't a oh I gotta have a 5.1 it was wait for about 30.00 more than a micro sound setup, I can get a deeper sound quality.
To Paul's point im not playing dig dug on this thing in mono.  Next time I build a pacman or something I will ensure it has only 2 paper cone speakers and baby amp to appease you.   
