Electronic – PCB plane cuts for audio paths in mixed-signal device

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.

Modern mixed signal ICs (audio processors and such - not op-amps and high end devices) mostly use a single rail power and their analog output is already centered around some DC voltage and does not go below zero, so with those kind of chips you can use the analog multiplexer as is.

But if you have a line out which is either floating or centered around zero volts, or can be something that you don't really know what it is (such as an external connector), then you must use a coupling (DC blocking) capacitor at the line out pin, and then use a "biasing" resistor to weakly pull the zero point to a suitable DC voltage between the rails. If your positive rail and ground are clean, then you can use a small resistor (something like 10K ohm) to both VCC and GND from the "other" end of the capacitor. Then you have an audio signal that sits nicely between the rails.

If the vcc (or avdd) signal is not clean enough, you must use a separate clean power source (a reference diode or good ldo) at a suitable voltage level to bias the signal to. For example, LDO output of 1.5 volts, and a 10K resistor from the signal to the LDO voltage.

At the other end you must use a similar blocking capacitor and just to be sure, weakly (100K) pull the other end to the common (ground) of the receiving line in. Depending on what the input impedance of the line in is (in the stone age it used to be 600 ohms but now it's rare to find such low impedance line in), you must determine the size of the coupling capacitor - it must be large enough that the bass frequencies don't cut off too much - because the coupling capacitor forms an RC filter together with the input impedance. 10 microfarades should be in the ball park.