Murmur is caused by blood passing through a narrow passage.
The following is for left ventricle, one can interprete the right in the same manner
During systole, left ventricle contracts. Bloot squirts out. Two possible places (generally speaking) are the supposedly open aortic opening and the supposedly closed mitral opening. If mitral opens and cause the noise than mitral regurg. If aortic valves not open completely, then aortic stenosis.

During diastole, left ventricle is relaxing and getting a new load. The blood can enter the ventricle from either the supposedly closed aortic opening and supposedly open mitral opening. If a murmur is heard, that means either the blood is from the aortic opening due to aortic regurge or mitral restriction blood (stenosis).
To distiguish the difference between the two areas you have to use other information such as hypertrophy, blood pressure, cardiac output, peripheral edema, pulmonary edema, etc.