In emacs24 -q,

    (require 'jack)
    M-x jack-start
    M-x jack-start

gets an error

    jack-maybe-rtlimits: Wrong type argument: stringp, nil

It seems if you don't have the set_rtlimits program then
jack-set-rtlimits-program is nil and in jack-maybe-rtlimits

    (file-exists-p jack-set-rtlimits-program)

doesn't like nil.  Perhaps add a check for that,

    (defun jack-maybe-rtlimits (args)
      (if (and jack-set-rtlimits
               (or (member "-R" args) (member "--realtime" args))
               jack-set-rtlimits-program
               (file-exists-p jack-set-rtlimits-program))
          (append (list jack-set-rtlimits-program "-r") args)
        args))

Rumour has it ( http://www.physics.adelaide.edu.au/~jwoithe/ )
set_rtlimits was renamed to set_rlimits, if it was worth trying that

    (defcustom jack-set-rtlimits-program
      (or (executable-find "set_rlimits")
          (executable-find "set_rtlimits"))
    ...

Though rumour also has it that for debian some PAM setups can do it more
easily for the whole audio group or something like that.