Hi, exactly what sox command are you giving? I'm on my phone till tomorrow and can't check, but the idea with jack is that sox will expose some output ports which you should connect, with qjackctl for example, to your soundcard.

Also, I was using sox too but was reccomended by many here ecasound, and for the little I used it it did indeed seemed better (audio flow is more clear)

Have fun,
Renato