Active networks have been recently highlighted as a key enabling technology to obtain immense flexibility in terms of network deployment, configurability, and customized packet processing. However, this flexibility is often achieved at the cost of router performance. In this paper, we present a three-level node architecture that combines flexibility and high performance of network nodes. We design and implement an active network application for real-time speech transmissions on top of this three-level platform. In our application, plug-in modules are downloaded onto certain network nodes to monitor packet loss rate of voice streams and to perform application-specific packet processing when necessary. In particular, we propose to perform loss concealment algorithms for voice data streams at active network nodes to regenerate lost packets. The regenerated speech data streams are robust enough to tolerate further packet losses along the data path so that the concealment algorithms at another downstream node or at the receiver can still take effect. We call our approach reactive concealment for speech transmission to distinguish it from concealment performed at the receiver and also proactive schemes like Forward Error Correction. Our approach is bandwidth-efficient and retains the applications’ end-to-end semantics.