A heart murmur that is present when a cat is born may be quite loud but of little significance. A heart murmur that was not present when a cat was younger is usually more significant. In general, I think it is a good idea to ask for referral to a veterinary cardiologist, or at least a veterinarian who can do a good job of cardiac ultrasound exam, to discover the significance of a murmur in a cat. This is the best way to determine how important a murmur is in a cat. Most vets use a scale of 1 to 6 to describe the intensity of a murmur.
Cats sleep a lot.
Up to 16 hours a day of sleep is considered to be normal for cats. But they shouldn't be lethargic when they are awake. If your cat isn't curious about what you are doing, doesn't come to check out what is happening when the can opener runs, won't play when it seems like he should, then lethargy is likely. Lethargy or tiring easily, associated with a heart murmur, in a cat, is highly suspicious for cardiomyopathy or an another heart problem that is causing significant problems. Ask your vet if there is a veterinary cardiologist or veterinary internal medicine specialist in your area who can do a good ultrasound exam of your cat's heart.