Sites for Auscultation
The bell of the stethoscope is better for detecting lower frequency sounds whilst the diaphragm is better for higher frequencies. The bell is usually used to listen to the mitral valve and the diaphragm at all other sites. Auscultation is usually performed with the patient sitting up or reclined at about 45°. Where variations are required, they will be described.

The best place to hear the heart valves is not necessarily directly over the anatomical site.
Mitral area:

At the apex beat, as the left ventricle is closest to thoracic cage.

Tricuspic area:

Inferior right sternal margin is the point closest to the valve in which ausculation is possible.

Pulmonary area:

Left second intercostal space close to the sternum is where the infundibulum is closest to the thoracic cage.

http://www.patient.co.uk/showdoc/40000504/
Aortic area:

Right second intercostal space close to the sternum is where the ascending aorta is nearest to the thoracic cage