Embouchure
Because of the fixed position of the windway with respect to the labium, fipple instruments can make a musical sound without the kind of embouchure required with (for example) the flute. However, it is not true that no embouchure is required to make a beautiful-sounding tone.
Embouchure on fipple flutes is centered around the idea of focusing the air inside the instrument's windway and bore alike, following the shape of the bore. Thus, a bore with a wide "bell" at the bottom of the instrument (as with Renaissance recorders) responds best to holding the throat wide open, to direct the airflow in a wide current so as to resonate the entire length and width of the bore. A bore which tapers down to a narrow "bell" (such as in Baroque-modeled recorders and school instruments) sounds best when the lips are used to focus the air to a tighter stream, to focus the air to the narrower "bell" at the bottom of the instrument. The idea is to always resonate the full length and width of the bore with the airflow, focusing the air accordingly. At all times, closing the lips around the "beak" of the recorder or fipple flute will help to focus the air down the narrow windway. This is very important to tone production on any fipple flute.
While a tight seal between the lips and the "beak" of the recorder focuses the tone, a tight facial musculature will also produce a raspy sound (with recorders, specifically). The combination of a clean seal with the lips around the beak, with the relaxing of the cheeks and face muscles, while allowing the cheeks to puff out in response to the flow of air, will be ingredients in the greater recipe of factors which produce a focused, ringy tone. This "greater recipe of factors" includes not only embouchure, but posture, articulation, breathing and fingering technique alike. Care should be taken not to block the windway with the teeth, which filters and scatters the airflow, producing a less-than-focused sound with a fuzzy edge, so to speak.
Read more about this topic: Fipple