jfet preamp output attenuated and clipped on bottom rail when connected to codec
From page 3 of the datasheet, it appears the input resistance of the codec is between 15k-30k (nominal, depending on the gain setting). This is of the same order of magnitude as your JFET amplifier's output impedance, so the signal reduction is consistent with what you have observed.

Regarding the clipping, the datasheet states that the input signal must be AC coupled. Your circuit is not doing this as P2 provides a bias current path to ground. This will be affecting the internal bias of the codec and is why it is clipping more on the ground side than the positive.

Putting another coupling capacitor at the output of P2 will fix the offset problem, but if you want the gain to be at all stable, you will have to either reduce the output impedance of your amplifier or add a buffering stage.

However, did you notice that the codec has an internal programmable amplifier? Why don't you just use a JFET configured as a source follower (unity gain buffer) and use the codec chip to scale the signal to match the ADC's dynamic range.