Hi,
I'm happy to hear that since 0022, the rMP3 shield is fully compatible.
I've been playing with it a bit, plugged to 2 small 8ohm speakers in series (to match the 16 ohm output impedance, and my sound file is mono): the output sound is very low, especially for bass.
Before thinking about how to add an amplifier, I wanted to try the volume and boost settings of the rMP3.
from RogueMP3 Arduino Library - Rogue Robotics :
setvolume(byte newvolume) - sets the volume (both left and right to same value).
setboost(...) - sets the audio boost enhancement.
hmm ... I managed to understand from different sources that setvolume(0) is max output, and that setboost works something as :
setboost(bass_amp [0 - 15], bass_freq [0 - 15], treble_amp [-8 - +7], treble_freq [0 - 15])
Is that right ? what bass/treble frequency range are covered by the [0 - 15] range ? how do the amp values work ?
In a more general perspective, I trust in rMP3 as a very useful and handy device (I bought 3 for my project), but now that it is fully compatible, is it not time for a synthesized documentation, in a single, linear document ? I know everybody's busy, but I have the feeling that at the present moment, important bits of info are spread over in the rogue wiki, which can be an obstacle for "base" users like me.
bhagman, if you agree to put together this "simple" user manual, I can offer some of my time if you feel the need for a french translation. 
What do you guys think ?
Vertigo.