Based on the ADG2128 analog crosspoint switch, the DS1881E for volume control, and the LME49726 as output buffers.

Features:

• 12 input channels that are arbitrarily routable to any of 8 output channels.
• Onboard digital potentiometers on the input side with non-volatile storage. Pots are logarithmic with zero-cross detection, and are paired in order with their input numbers on the board's silkscreen.
• Onboard op-amps on the output side with 300mA drive capability to provide buffering for direct-drive of line-level signals.
• Onboard buffered voltage divider with exposed output and 300mA drive capability to aid design and implementation of input buffers.
• Resistors added to the input-side of the switch, but after the potentiometers to allow for simple resistance mixing of signals simply by routing two inputs to a given output without risking damage. Some caveats apply (discussed below).
• Input-side is left free-form to allow use with single-ended (unbalanced) or differential (balanced) signals, and/or DC-biased sources.
• Safe for 5v audio signals (see notes).
• Isolated analog and digital power planes.
• Silkscreen reflects the envisioned usage (stereo signals), but the driver allows for easy grouping of any number of channels.
• Breadboard-friendly

Notes on contentious routing:

Any mixing effects between two input channels is accomplished as a simple resistance mix with the switch as the intermediary. The maximum current through a given channel must never exceed 1mA. The POT wipers are the weak-link. The resistance between two input channels connected to a common output channel is: - twice the switch connection resistance (typically a few hundred ohms at 5V), plus - twice the 330 ohm series resistance, plus - the values of the wipers in the circuit (or the wiper to VccA/2)

Input stage should be fed single-ended and biased by the VccA/2 pin for safety, as well as for zero-cross detection. Recommended circuits are in the documentation.