The Collaborative International Dictionary of English v.0.48:

Speakable \Speak"a*ble\, a.
   1. Capable of being spoken; fit to be spoken. --Ascham.
      [1913 Webster]

   2. Able to speak. --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

speakable
    adj 1: capable of being uttered in words or sentences [syn:
           speakable, utterable]