You can send option value by passing this.value to javascript function on change event.

<form action="">
  <select name="channel" onchange="sendCommand(this.value)">
   <option value="#channel1">channel1</option>
   <option value="#channel2">channel2</option>
  </select>
 </form>

Or using jQuery:

$('document').ready(function() {
  $('select[name="channel"]').bind('change', function() {
    sendCommand($(this).val());
  });
});

You can directly wire the event with jQuery and concatenate JOIN to it.

$(function(){
   $("select[name='channel']").on("change", function(){
     swfobject.getObjectById('lightIRC').sendCommand("JOIN "+ $(this).val());
   });
});

Hope this helps