Boss is completly correct. There are different grades and there is also the issue as to whether they were born with them or whether they develop throughout life.
Mine has a heart murmour he was born with and has passed 3 5 star vettings, one which was with the purpose of affiliated eventing.
Its there but its never worried me at all and its never gotten worse (hes 16 now)
What about a grade 4 murmer?
Horse in question played quite a hard fast sport at a high level (so had to be very very fit) and never had a problem apparantly.
4, thought, was quite severe. I was told 5 was at the point of it being none recommended to ride so i would have *assumed* 4 would be where they dont recommend hard or fast work.
My lad Gedenski has a grade 3/4 murmur, which he's had since birth. It didn't stop him winning and placing in several 100 mile in a day endurance races, completing over 6000kms in his career and is still going strong today - he's 20 in April. So his murmur has certainly never limited or curtailed his competing - you don't get anything tougher on the horse than a 100 mile race ride
I should add though that I believe it depends on what sort of murmur it is - i.e. which heart valve it involves