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.