I can't play some songs that I purchased in my Sonos player because they are protected. What can I do?

Upgrade them to iTunes Plus.

tt2

I'm trying to do this, but I don't see any Itunes Plus link in the current version of Itunes.

The upgrade links seem to come and go. What happens if you try to purchase one of the tracks you have in protected format? You may be charged a different price.

tt2

This migth help https://discussions.apple.com/thread/1495590