But, with age or due to genetics, the heart valves can become defective or have lesions causing them to not fully close. That results in a "leak" whenever the heart contracts, so the vet hears a murmur. The louder the murmur, the more blood is shooting backwards.
Right now, we don't know the grade of the murmur or the severity. A veterinarian is supposed to grade how loud they think is the murmur on a scale of 1 to 6.
Typically, if there is a new murmur, I recommend some chest x-rays first. Why?
Well, chest can show if the heart has had to compensate for the valve problem. The heart will literally change in size and/or shape.
Can you change the murmur? No
You first need some additional testing like the chest x-rays, maybe a blood pressure, and sometimes a heart ultrasound (echocardiogram) to confirm the cause of the murmur.