I'm assuming a vet told you your cat has a heart murmur. There's a whole range of severity of heart murmurs. Indeed, there's a whole range of interpreting heart sounds as murmurs. You should discuss the implications of the diagnosis with your vet. Only he/she knows what they are. Many cats live a normal life with that diagnosis. And discuss your cat's behavior change. There's a good possibility it has nothing to do with the heart murmur.