Having a jackd service is not necessary (SuperCollider will start one if
it's not already running), but in case you'd like to have one I've found
the following to work with Shepherd:

--8<---------------cut here---------------start------------->8---
(define jackd
  (make <service>
    #:provides '(jackd)
    #:requires '()
    #:start (make-system-constructor "jackd -d alsa &")
    #:stop (make-system-destructor "pkill jackd")))
--8<---------------cut here---------------end--------------->8---

Thanks,

Maxim