If your dog was born with this murmur
it may just be what is called an innocent murmur
and will be benign. As your dog
ages, all muscles in the body may become less toned. This reduction in tone includes the heart. The murmur may increase with age.
Your best bet for prognosis is to eventually have a cardiac workup including echocardiogram, EKG, chest X-Rays, and bloodwork. Heart medications, if necessary are very effective and some can even help to prevent further deterioration.