I don't know very much about heart murmours, but I do know my parents' Cavalier had one, and lived a long and active life - my Dad even walked her up the fells in the lake district. It was her heart that eventually gave out, but that was at a good old age (for a Cavalier, at any rate!) and although she was on some heart medication towards the end of her life, it really did not impact upon her quality of life in the slightest.
I think quite a lot of dogs can have a slight murmour - it's something to keep an eye on, but no cause for major panic.