Better Health has a good explanation of this:
I've read a lot about Heart Murmurs (especially since I was diagnosed with an Innocent Heart Murmur.) They can happen in anyone of any age. Apparently innocent Heart Murmurs are commonly heard in children, as their heart is so close to their chest.
I went for all these tests of my citizenship. The doctor found an innocent heart murmur. Innocent means harmless, the heart structures and blood flow are normal - just noisy. No treatment is necessary. I went for echocardiagram, everything was fine. My doctor reckoned that it wasn't there in the first place. I was stressed that day, which can be a factor.
This page has good information about murmurs:
Not sure about echocardiograms and sonograms. An echocardiogram is an ultrasound of the heart. Stethoscope aren't 100% accurate, so if you have symptoms it's good to get an echocardiogram. I didn't have any symptoms. But having an echocardiograms made me feel better about the whole thing.
The probability of a serious heart murmur in young people is very slim. If the doctor said that it was innocent, don't worry. If he didn't say what he called the murmur, or called it something other than innocent. leave it up to the echocardiogram to find out if it does exist.