Heilan is a cross-platform OpenGL X3D browser written in C and designed for audiovisual performance. Specifically, it forms the environment within which I developed an audiovisual instrument (Ashitaka) for my PhD. As such, it has a number of features not commonly found in X3D browsers:
Low latency audio courtesy of PortAudio (ASIO, DirectX, MME on Windows; Jack, ALSA, OSS on Linux; CoreAudio on OSX).
A 1st order B-format Ambisonic audio engine, allowing for full 3d sound, capable of accommodating virtually any speaker configuration.
Open Sound Control support for all nodes, allowing a node's attributes to be manipulated in realtime, potentially by multiple users.
A multi-threaded audio engine, able to split off different nodes' audio processing into separate threads (and potentially, separate cpus/cores). This can be configured by the scene author.