Downstream of the vocal folds, it is often assumed that all the kinetic energy of the glottal jet is dissipated by turbulence. Although this assumption is reasonable for normal voicing, there are many configurations for which a constriction or even an occlusion of the vocal tract occurs and leads to a significant pressure recovery. Such a phenomenon occurs during the production of vowel-bilabial plosive sequences. In this paper, we will first present results from measurements on a human speaker during production of vowel-bilabial plosive-vowel sequences. These measurements concern intra-oral pressure, acoustical pressure, EGG and lips video recording of a female subject. The relative timings between the motion of the lips, the glottal signals and the intra-oral pressure are analyzed and related with Voice Onset/Offset Times. A theoretical model for air flow in the lips is validated by experiments on a mechanical replica of the vocal tract including a self-oscillating glottis and a controlled moving constriction to mimic the closure of the lips. This theoretical model is used to achieve numerical simulations of an /apa/ utterance. The differences between the intra-oral pressure observed in vivo and the simulated one lead to an investigation on the influence of the cheeks expansion during bilabial plosive production.