One of my siamese boys was diagnosed with a heart murmur at 6 months. He was scanned and there was no evident problem.
He is now three, and since that time, no murmurs have been heard!

I am not impressed that your vet scared you like that! Many kittens have murmurs which often turn out to be nothing, or to resolve themselves. Have a scan done and that will tell you what you are dealing with. If it does turn out to be a problem, there are many treatments available!
Hope all turns out well for you.