Would like to request support for routing the output from a VM guest to JACK. This would allow using pro-audio software only available on windows or mac and having the audio input / output interact with Linux pro-audio apps. It might be more complex to pull off than support for other audio systems since JACK audio is geared towards low latency,
Not sure what is supported on the windows guest side of things, but I would imagine that it would need a virtual device that uses an ASIO driver. (And I don't know if the current virutal audio devices presented to guest OSs have any Windows ASIO drivers for them.)
If you have questions about implementation, consult the Linux Audio Developers mailing list.