[namibj]

You should be able to use seL4 as a hypervisor and stuff a GNU/Linux system inside. The actual low-latency work would be done via native seL4 processes. It's proven to have hard latency bounds, thus being suitable for hard-realtime applications (except for modern x86_64 CPUs having special interrupts that can't be disabled, and thus possess the capability to introduce latency spikes of potentially unbounded duration). The HFT community found ways around those issues, however. It wouldn't be good enough to control a manned aircraft, but for entertainment-related audio, it should easily be good enough (those spikes are around a millisecond or so, iirc).