The Collaborative International Dictionary of English v.0.48:

Hark \Hark\ (h[aum]rk), v. i. [OE. herken. See Hearken.]
   To listen; to hearken. [Now rare, except in the imperative
   form used as an interjection, Hark! listen.] --Hudibras.
   [1913 Webster]

   Hark away! Hark back! Hark forward! (Sporting), cries
      used to incite and guide hounds in hunting.

   To hark back, to go back for a fresh start, as when one has
      wandered from his direct course, or made a digression.
      [1913 Webster]

            He must have overshot the mark, and must hark back.
                                                  --Haggard.
      [1913 Webster]

            He harked back to the subject.        --W. E.
                                                  Norris.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hark
    v 1: listen; used mostly in the imperative [syn: hark,
         harken, hearken]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "hark":
   NB, attend, attend to, auscultate, be all ears, bend an ear, bug,
   cock the ears, eavesdrop, examine by ear, give attention,
   give audience to, give ear, give ear to, hear, hear out, hearken,
   hearken to, heed, intercept, lend an ear, listen, listen at,
   listen in, listen to, mark, mind, note, notice, remark, sit in on,
   tap, wiretap