Change the default alsa sample rate to 44100

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
by setting
defaults.pcm.dmix.rate 44100 (default is 48000)

See this thread at the raspyfi forum.

Are you using Pi’s jack out?

No, a USB dac.