When your heart is beating, a number of valves stop the blood from flowing backwards and going out the wrong way. The heart is designed to make sure that it keeps pumping blood forwards and get it flowing round the body, not going backwards!! However, whenever these valves shut (to prevent the backflow of blood) they create a bit of a racket; that is, they cause what is known as a heart sound.
You can't actually hear the heart sound from outside the body - the sound itself is actually just made by the vibration caused. When the valve shuts, it's like shutting a door - you can sometimes feel it on your face because shutting a door creates vibrations in the air. Alternatively you might appreciate that if you put something in the way of flowing water, it causes the water around that blockage to become disrupted, and the flow is changed. These vibrations which result from the valve closing are picked up on the surface by a stethoscope as heart sounds, and depending upon their timing, can give clues to what's going on in the heart.
The normal sound that you'll hear through a stethoscope is commonly described as 'lub-dub', with lub being the first heart sound and dub being the second. If you have a listen to a real heart, the description is helpfully similar to what you hear.
The first heart sound is made by the atrioventricular valves - that is, the heart valves between the atria and the ventricles. The main one is the mitral valve, between the left atrium and the left ventricle.
The left hand side of the heart is more forceful because you've got to get the blood all around the body. This means that with any heart sound, you're more likely to hear the sound made by the valve on the left than the right. Even though the tricuspid valve is shutting at the same time as the mitral one, it's the mitral one that you'll hear through a stethoscope.
The volume of the first heart sound, or S1, depends on how forcefully the mitral valve shuts. Something which causes the mitral valve to shut very forcefully, like mitral stenosis, is going to cause a loud S1. On the other hand, if the left ventricle takes a while to fill or to eject its contents (e.g. aortic stenosis, or left bundle branch block), then the sound made by the mitral valve shutting will be softer. If you have a good ear for these things, then the chance are you may be able to pick up a lot simply by the volume of the first heart sound!
As mentioned, S1 also comes from the tricuspid valve closing. If the mitral valve is completely normal, you may still get a loud S1 because you have, for example, tricuspid stenosis. In that case, the loud S1 is coming from the right side of the heart. However, normally you'd assume that the vibrations picked up by the stethoscope are from the left. When you are examining the cardiovascular system, you'll find that the first heart sound will probably be loudest around the area it's coming from.
The second heart sound is made by the valves between the ventricles and the great vessles - that is, between the left ventricle and the aorta, and between the right ventricle and the pulmonary artery. The main one is on the left hand side again - the aortic valve.
Once again, the volume of the sound (in this case S2) will depend on how forcefully the valve shuts - so if there's high pressure across the valve, you could find it shutting loudly. This is found when there is hypertension (high blood pressure). Once again the sound is made up of two valves shutting, the aortic valve and the pulmonary valve. If there is pulmonary hypertension, the sound made by the closure of this valve will also be louder.
One of the interesting things about the second heart sound is that it can become 'split', even in normal people. If people are young, so their body is responding very well to different situations, then the aortic and pulmonary valves may close at different times. This happens particularly during breathing. When you breath in, the pressure drops inside the thorax causing more blood to come to the right side of the heart. The increase is not felt on the left side of the heart because the blood collects a bit more in the pulmonary veins. This means the volume on the right is slightly bigger than on the left, and it takes longer for the right ventricle to squeeze all the blood out - so the pulmonary valve closes ever so slightly later. This is enough to 'split' the second heart sound, causing a lub da-dub sound which returns to normal when you breathe out again.
If you're listening in to the heart sounds and they don't sound like a normal lub-dub then either you've got a heart murmur, or you've got added sounds. Added sounds are split into two categories - splitting of normal heart sounds, or the addition of extra sounds altogether.
If the valves don't close in time with each other, you getting splitting of the heart sound. The first heart sound can sometimes be split in normal people, but it can be associated with illness such as a disorder of the heart's electrical system called right bundle branch block. The second sound can also be split normally, as already described, but you must make sure that it's not because the left or right ventricles are taking too long to empty. In right bundle branch block, pulmonary stenosis or mitral regurgitation, right ventricular emptying can take longer leading to the pulmonary valve closing late; in left bundle branch block, aortic stenosis and coarctation of the aorta, left ventricular emptying can take longer leading to the aortic valve closing late.
S3 is an extra sound which, if it exists, sounds just after S2. In young people it can be normal, but if it happens at an older age then it suggests something's wrong. It happens when blood fills the ventricle very quickly, with blood rushing into the ventricles as soon as the atrioventricular valves open - either because there's a problem with the ventricle (e.g. dilated cardiomyopathy), or because there's too much blood swirling around (e.g. because of regurgitation of the mitral or aortic valves). It's easier to hear with the bell of a stethoscope because it's a low frequency sound, and it'll sound like ken-tuck-y instead of lub-dub.
S4 is an extra sound which, if it exists, sounds just before S1 and you don't need to have S3 in order to have S4 - it's still called S4 even if there isn't any S3!! It always suggests something's wrong, because it suggests that the walls of the ventricles are too stiff and the atria are having to contract a lot harder to get the blood into them. The two main things to be thinking of are high blood pressure and hypertrophic cardiomyopathy. This time it'll sound like ten-ne-see instead of lub-dub.