A system that is distinct from a remote server and distinct from a client device receives, over a first communications channel, a first data stream for a first media item. The system receives, over a second communications channel, from an application at the client device, a second data stream for audio data that includes vocals provided by a user as the first media item plays. The system measures a latency of the second communications channel and overlays, with the first media item, the vocals provided by the user as the first media item plays to generate a composite data stream. The overlaying comprises offsetting the first data stream from the second data stream in accordance with the measured latency of the second communications channel and combining the first and second data streams in accordance with the offset of the data streams.