How to vote for both answers and question using the Answers module?
Instead use Question/Answer module, it have the vote system for both question and answer


  It adds a question/answer section for website. Different from other
  modules, it uses only one node type for question, and the core
  comment.module for answer. This allow reutilisation of CCK, Views..


or

You can try Vote Up/Down module


  Allows votes on some drupal entities and provides the base for
  implementing votes on other entities


or take a look at Rate module 


  This module provides flexible voting widgets for nodes and comments.
  Multiple ratings on the same node (i.e. for "comfort", "location",
  "services" etc.)