Routing audio signal with CMOS Analog Multiplexer
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.