I have done/tried a set-up using the SimpleSDAudio lib as the audio output.
the most simple design, straight outout put on pin9 via a 100uF polarized cap.
the pwm 'works'... but you get noise in the audio.
Specs:
ATmga328P-AU
+3.3v @ 8MHz internal clock (ie; no crystal, cap or resonator)
straight connect microSD (ie: no level shifter or voltage divider)
3 x leds on PWM fade
5 x leds (digital on/off)
no filter
no amp
(I need to learn more about low-pass filter set-up and using an amp better) 
@cobra18t-
is there any schematics or anything for your BabyBOOMER?
Does it need the 'dual' output? I only have set-up where Im outputting on pin9 only (most basic)..
looking for the smallest way to get better sound.. 
thanks