It is simpler to make a correct diagnosis when you have an appropriate experience. If you have extensive experience, you can make the diagnosis almost automatically without any mental strain. 
It 's hard to make a diagnosis when you meet some disease for the first time, especially when this disease is rare. That is why it is useful to read good descriptions of clinical cases. Here is another one challenging case. 

A patient is a man in his fifties. Many years ago in his childhood, he had some heart surgery to treat some congenital anomaly, but there was not any medical records about it except one little yellow certificate about some congenital pulmonary trunk anomaly. There was congestive heart failure: shortness of breath, oedema, peripheral cyanosis, ascites. ECG: sinus rhythm with frequent premature ventricular contractions. 

Here is his heart’s sound, apex, left decubitus position:

I was listening to his heart and finally understood nearly nothing. Differentiating neither S1, nor S2 was impossible. Heart rhythm was irregular. There were groups of unclear sounds and murmurs divided by pauses. I heard this recording some tens of times, I made many visualisations of this sound (when I was preparing this publication, of course), and finally, I was not able to interpret this sound. But there was the only key: the final sound in every group was a low-frequency murmur. I separated this murmur, listen:

In my opinion, there was only two explanations of this murmur:
- mitral stenosis
- pulmonary regurgitation without pulmonary hypertension

Mitral stenosis is very familiar to me because I met it many hundreds of times. But I never sow pulmonary regurgitation with such loud murmur. So, it seemed that it was mitral stenosis. 

But it was severe pulmonary regurgitation. Watch the video below:

Severe pulmonary regurgitation is the relatively rare disease. In most cases, it is related to many congenital anomalies and, especially, to surgically treated pulmonary stenosis. Pulmonary artery’s pressure is low. That’s why diastolic pressure gradient between pulmonary artery and right ventricle is small. In such case pulmonary regurgitation murmur is low-frequency. Similar to murmur of mitral stenosis.

When pulmonary hypertension is present, the pressure gradient between pulmonary artery and right ventricle is high. So, the murmur of pulmonary regurgitation with pulmonary hypertension is high-pitched. Like murmur of aortic insufficiency.

Severe pulmonic insufficiency has wide range of clinical presentation. The complete congenital absence of pulmonary valve found in the seventh decade of life was described! It means that life is possible without pulmonary valve at all! It is impossible for aortic valve, by the way. 

Learning point: this case is remarkable because of lack of any possibility to interpret heart sound due to arrhythmia. Simultaneously recorded ECG would be helpful. But the terminal sound in all ‘chaotic’ clusters of heart sound was confidently interpreted like diastolic low-frequency murmur. This finding made differential diagnosis list much shorter:
- Mitral stenosis
- Pulmonary regurgitation