Hi Peter, using an external USB ADC to connect turntable to Volumio, I get same observation (48kHz for a 192kHz capable ADC).
look at my thread here : Vinyl lovers : ADC I2S card as Volumio analog input - driver compiled but no ALSA mount - #10 by BenoitMarhieu
I think the 48Khz is the default frequency of ALSA and the current implementation of non I2S sources in Volumio has not been nailed down to their full capabilities.
My gut feeling after some search is there are ways to manually define card capabilities in ALSA, but this is currently by manual change in the files, not through Volumio.
@volumio , @dvo
when all this will be sorted out, I think it would be good to write a case as more and more people are struggling in understanding and properly setting up this DIY analog input capability (I know we could buy the Primo and get it working 
)
Benoit