Sorry this is going to be a long reply
I have a murmur, they found it when I was 14 and I was probably born with it. It is usually caused by a slight malformation in one of the heart valves (usually the mitral and more rarely the tricuspid). The malformation is often just an extra piece of tissue on the flap that closes the valve, so that when the valve slams shut, the flap goes fwop and makes a sound that they can hear with a stethoscope.
Having a floppy valve (more properly called a prolapsed valve) is most often completely benign, no different to being born with a birth mark etc.
Occasionally it can mean that the valve doesnt seal completely when it slams shut and so some blood can leak backwards instead of it all being pushed forwards into the system. This is actually the murmur (but many GPs will call the sound of the valve flopping a murmur as they are not trained enough to detect the difference between the fwop and the backwash sounds). Murmurs can be mild, moderate or severe depending upon the amount of blood that backwashes. I had no backwash with my valve until my third pregnancy when I developed a mild murmur. It has no impact on my life or ability to do anything. If I do progress to a moderate or severe murmur, then there are steps they can take to fix the valve by going in through a vein with a wire and doing a micro surgery (very cool new age stuff)
Having a prolapsed valve can also put you at slightly increased risk of heart arrythmias called PVCs and SVT. Both are benign at low levels and mostly just annoying.
So your DC should see a cardiologist and get an ECG and more importantly an Echocardiogram of the heart (like an ultrasound but of the heart not fetus). That will be able to tell you that the structure of the heart is normal, if there is a prolapse and where it is, if there is backwash and how much it is. Given your DC has no symptoms I think it is highly highly unlikely that there is any problem beyond a benign flap in the heart that will cause no long term problems. But it is important to get it checked so that you know about it.
If you would like more info please pm me.