The default alsa sample rate is normally 48kHz. Since shairport outputs 44.1 kHz alsa resamples the shairport output to 48kHz which will have a negative effect sound quality. Since the goal with volumio is to have the highest possible sound quality I think the default sample rate should be changed to 44.1kHz.
This is done in /usr/share/alsa/alsa.conf
defaults.pcm.dmix.rate 44100 (default is 48000)
See this thread at the raspyfi forum.