Audible atrial flutter sound creates a strange impression of two simultaneously beating hearts in one chest cavity. It is a rare and subtle sign. Here I will show you two cases.

Note, while atrial flutter atrial rate is 250-350  bpm. Thus, atrial flutter sounds repeat with high frequency and could be heard only when high-degree AV-block with normal heart rate or bradycardia is present. These sounds could be heard on apex or the base of the heart. 

A male patient about his fifties with old anterior Q-MI, atrial flutter and heart failure, apex (two recordings):

Here is a visualisation of this sounds:

We hear a loud S1, arrhythmia and soft low-frequency "distant" fast sounds. When I was examining this patient some years ago, I was impressed. It seemed there were two hearts in one patient: one close to my ear, and another far away. When I am not able to interpret quickly challenging auscultatory findings, I try to auscultate more until I realise everything. It took about ten minutes when I finally understand I heard atrial flutter sound.

It is nearly impossible to say how frequently atrial flutter produces audible sounds. I think in most cases these sounds are too subtle and are missed.  On the other hand, it is a really rare sign.

Some years after I was listening to my collection of heart sounds. While listening to the recording of the heart of a female patient with severe mitral regurgitation, I noticed some fast sounds which were missed initially. Here is this recording:

You can here S1, S2, systolic murmur, arrhythmia and repetitive high-frequency atrial flutter sounds:

It is interesting to hear soft sound separately and maximally loud. Here is it! I cut atrial flutter sound and made it maximally loud:

This is a visualisation of three consecutive atrial flutter sounds, which was cut and made loud:

A generation and significance of audible atrial flutter sounds are unknown. 

P.S. Bonus: I made some fantastic sound design using atrial flutter sound only: