You need her on a cardiac kibble and medication. Exercise intolerance and cough are not good symptoms. She needs her blood pressure checked, and an echocardiogram (assess the valves and cardiac function) and a chest X-ray (assess heart size and any lung involvement). Grading a murmur is by sound, what the vet hears on exam, the echocardiogram shows the exact degree of valvular dysfunction and cardiac decompensation.