Although lip-smacking makes a quiet sound (similar to "p p p p"), it is not accompanied by phonation, which is produced by vocal cord vibration in the "voice box" or larynx.
Although superficially lip-smacking appears to involve simply rapid opening and closing of the lips, the x-ray movies show that lip-smacking is actually a complex behaviour, requiring rapid, coordinated movements of the lips, jaw, tongue and the hyoid bone (which provides the supporting skeleton for the larynx and tongue). Furthermore these movements occur at a rate of about 5 cycles per second, the same as speech, and are much faster than chewing movements (about 2.5 cycles per second). Thus, although lip smacking superficially resembles "fake chewing," it is in fact very different, and more like speech
D - making P and not B the natural default. Maybe growling on top produced the first B voiced P?