use elem.value instead of DOM attribute getter for audio api change

pd/nw/dialog_prefs.html

@@ -401,8 +401,9 @@ function cancel() {
 }
 
 function change_api(elem) {
+pdgui.post("elem is " + elem);
     var id = elem.getAttribute("id"),
-        value = elem.getAttribute("value");
+        value = elem.value; // need js object property, DOM Attribute won't work
     if (id === "audio_api") {
         pdgui.pdsend("pd audio-setapi", value);
     } else {