Finally I have got the new circuit designed and the PCB has arrived! Here is the board:

And it built:

Finally it works! The unit is able to take the push buttons on the front and select the right input. I have also used 6x2 phono connectors rather than individual connectors as I have used on my amplifier. These are cheap versions and you can see that the 1st port (the output) has fallen out. You can see a small tang that needed to be pushed 'up' for it to lock in correctly. Not seen in the above photo is the second power connector and the I2C connector (a 3.5mm stereo jack). The I2C port allows the unit to be controlled from the amplifier as well.

You may notice that the crystal (and associated capacitors) are missing. This is a cost saving exercise as the ATMEGA doesn't need the timing precision to do it's job. My original idea was to use the internal RC oscillator so not sure why I put the crystal on there!

I'll see about getting the new PCB and Arduino code uploaded soon.