It can be very hard to determine if a heart murmur is present when listening to the chest of an excited dog. Respiratory sounds can mimic a heart murmur when respiration is rapid enough to approximate the heartrate.
Heart murmurs vary widely in their importance. Hearing a heart murmur in a dog that seems normal in all other respects doesn’t usually make me want to pursue a lot of immediate testing to determine the cause but it does make it important for the dog owner to watch carefully for signs of heart failure, such as tiring easily, coughing, weight loss and difficulty breathing. If any of these signs are present then it is much more important to try to identify the cause.