Oh I can see the setting you need in the settings->playback options menu item :
http://volumio.local/plugin/audio_interface-alsa_controller
At the bottom, enable “audio resampling”
You can play with the features, I think the best solution is 32 bits, very high quality.
96 kHz or 48 kHz depending on your needs.
As long as the Pi can handle the high sample rate and “high” or “very high” rate conversion, everything should be good.