Traditional multiparty audio or video conferencing uses a single node, sometimes called a multipoint control unit, or MCU, to mix audio data for the conference. We introduce a novel mixer, called a Virtual Mixer, which performs mixing in a distributed way over the network. The Virtual Mixer topology is optimized over Steiner trees using a metric of either average pairwise delay (APD) or maximum pairwise delay (MPD). Since the topology is adapted to the particular set of clients and servers available in the cloud, optimization speed is important. In order to solve this NP-hard Steiner tree optimization, we propose heuristic algorithms for finding the Pairwise-delay-optimal Tree (PT) for both APD and MPD, which are orders of magnitude faster than exhaustive search, yet find trees with delays that are minimal or within a few percent of minimal. We show through experiments both on a corporate intranet and on up to 12 PlanetLab nodes that Virtual Mixing can reduce both the APD and the MPD between clients by upwards of 50%, compared with the existing MCU-based and P2P-based mixing approaches.