I have had this issue since my SA V+.
As bluecandylover said, not all TVs pass 5.1 though the optical, such as my Samsung PS42C450.
If you want to stick with optical the cheapest option is a physical switch as you already mentioned. I have this one: http://www.amazon.co.uk/Ex-Pro-Optic...8378354&sr=8-8
It's as cheap and flimsy as it looks but does the job, requires a little jiggle to get it to line up occasionally but has done the job for 2 years now. Getting off the sofa for a few seconds now and then isn't such a bad thing
I'm now saving the pennies for an HDMI AV amp to replace the Samsung HT-C555 DVD/home theatre so I can achieve the dream of not having to get off the sofa to change audio input. Plus the Samsung HT-C555 is crap.