I've found the solution to my problem which turned out to be annoyingly obvious. The description of SuperCollider according to Guix reads: > … > SuperCollider requires jackd to be installed in your user profile and > your user must be allowed to access the realtime features of the > kernel. > …
It turns out that the jack-2 package doesn't provide the jackd command like I expected. Swapping this out for the jack-1 package (which provides jackd) fixes this. My fault really for failing to read the description, but I'm now wondering if SuperCollider can be modified to report when this command is missing; I can't imagine I'm the first person to run up against this. Interestingly also, the supercollider-server package on Debian/Ubuntu as provided by apt includes the jackd package as a dependency. Is there a reason SuperCollider on Guix doesn't or shouldn't depend on jack?
