I would not begin to tell you what to do about this but I will tell you that heart murmurs are extremely common, especially in children. Sometimes, they outgrow them or they may seem to come and go -- sometimes heard and sometimes not (that is the case with me). There are lots of articles like this one:
Most Murmurs Are Of No Concern - Teen Growth online that basically say most heart murmurs are innocent. But I would want the doc to tell me that it WAS the innocent type. Hoping the ecg gives a green light for sports and affirms an innocent murmur.