I am not sure but I can't find any words that describe normal calm breathing. Something that describes the exhale of a breath or blowing air out from your mouth. Not frantic or puffed breathing just relaxed sounds like a sigh. But a sigh describes what the person is doing... not really the actual sound... Are there any English words for this?
The most common word for breathing is "respiration" which is often broken down by definition into two actions - to inhale and exhale. There is also the technical term for normal breathing which is "eupnoea". However, the word "breathing" itself would seem to be descriptive enough for most usages.
If you are searching for an onomatopoeic word for the (sound of) regular breathing, I did find "susurration". It is a noun mostly used for a low, indistinct whispering sound. I found this online descriptive usage - susurration [a soft, whispering or rustling sound] of the breeze in the wayside grasses. However, it could be used as descriptive of breathing sounds as well - e.g. normal/calm/slow/indistinct breaths could be replaced with "susurrations" (the inhalations/exhalations of normal breathing in context). It can also be used in the adjective form "susurrous" - e.g. susurrous respiration/breaths/sounds.
I can tell you that, from the standpoint of someone administering first aid, the way to express what you're trying to express (the sound of normal breathing) is "calm breathing". This, however, would be clarified with a respiratory rate in terms of breaths per minute, because "calm" would depend on multiple factors: age, elevation, weight, and they would all correlate to a respiratory rate. Knowing the respiratory rate would be the best measure of normal breathing.
Now, it also sounds like you are trying to capture a sound to calm breathing, and this is merely a matter of perception. A medic would likely comment on the sound of breathing by noting audible elements: a rasp, a wheeze, a hiccup, which might indicate an abnormality. Or, they would note nothing abnormal at all, which is to say, stable breathing with no signs of a disturbance. And this would be noted as "calm breathing". That is to say, there is no single sound associated with calm breathing. We all breathe differently if you get down to the sounds we make.