We propose a synchronization control scheme which achieves both speech/video Intra-Stream synchronizations and Inter-Stream synchronization for videoconferencing services over IP networks. The driving principle of our scheme is to guarantee the Intra-Sync speech timing relationships (hence the speech quality) and to adjust the video Intra-Sync and the Inter-Sync accordingly. Towards this aim we use a preventive control for the speech stream and a reactive control for the video stream. More precisely, we use an adaptive playout algorithm that keeps the Intra- Sync constraints within the talkspurts, while the network jitters are compensated by modifying only the silence period lengths on the basis of both speech and video packet delays. We implemented our scheme in a prototype, which allowed us to test the effectiveness of our solution. Actually, we could appreciate both perfect speech intelligibility and very satisfactory user perceived lip-sync. The latter is because the Inter-Sync error is concentrated only at the beginning of the talkspurts, where known experimental tests have shown that it is not detectable.