I took a look on ebay.com whereas HDMI switch boxes were available boxes to switch Left and Right Audio on Phono jacks were rare and expensive
I wondered if I could use the TV remote and build a decoder to switch between the 2 sources. As I had a couple of Arduino Unos, bought when on offer at radioshack, they seemed an ideal candidate
Searching the internet I came across some web pages at adafruit.com that would be useful. A walk to radioshack (I am now only 300 yards from one now) procured the necessary IR detector.It was hooked up to the arduino and the software loaded from http://learn.adafruit.com/ir-sensor/reading-ir-commands. I pointed the IR remote that came with my RTL2832 TV dongle at the decoder and saw the pulse codes. The software allows the pulse codes to be recorded. and placed in the file you can use as a reference to switch arduino outputs. I choose the red button to choose TV as source and Green button for Computer.
Next a 6V DPDT relay was found in the junk box and mounted on an arduino protoshield board. The relay was switched with a 2N7000 FET from the arduino output. Two LEDs were added in software and hardware to show the audio source. The whole assembly was mounted in a Diecast box with a hole filed out in the front panel to allow the IR detector to be mounted.
The arrangement worked fine but I kept losing the RTL2832 controller, which was small. I decided to use two of the spare buttons on the existing IR controller, Not hard, all I had to do was record the pattern and put it in the lookup file
A useful afternoons work!