I suggest to include soxr in the MPD for upsampling. I built it on my RapsyFi and it resamples 16bit/44.1kHz smoothly into 16bit/96kHz.
I also have tried using pipe output and stand alone sox for upsampling on my RaspyFi before. It can barely run with the default setting but cost twice CPU resource than simply using soxr. I believe soxr is the audiophile resampler Volumio cannot miss.
Lastly, I would like to have the pipe output option of mpd. It is useful for someone like me who want to play around with the sound using other softwares, such as ecasound.
I have not build soxr or mpd on Volumio yet and will report what I find afterward.
Thanks much for such a great distribution.