Added title and alt tags to the string that is used in the question

editing form. I did not know what the icons meant as they are not
right next to the dropdown in the PRT part of the form.

lang/en/qtype_stack.php

@@ -336,8 +336,8 @@
 $string['quiet'] = 'Quiet';
 $string['quiet_help'] = 'When set to yes any feedback automatically generated by the answer tests is suppressed, and not displayed to the student.  The feedback fields in the branches are unaffected by this option.';
 // The icon fa-volume-off isn't very good really.
-$string['quiet_icon_true']  = '<span style="font-size: 1.25em; color:red;"><i class="fa fa-microphone-slash" aria-hidden="true"></i></span>';
-$string['quiet_icon_false'] = '<span style="font-size: 1.25em; color:blue;"><i class="fa fa-commenting-o"></i></span>';
+$string['quiet_icon_true']  = '<span title ="Quiet on" alt="Quiet On Microphone icon" style="font-size: 1.25em; color:red;"><i class="fa fa-microphone-slash" aria-hidden="true"></i></span>';
+$string['quiet_icon_false'] = '<span title ="Quiet off" alt="Quiet Off Microphone icon" "style="font-size: 1.25em; color:blue;"><i class="fa fa-commenting-o"></i></span>';
 $string['renamequestionparts'] = 'Rename parts of the question';
 $string['requiredfield'] = 'This field is required!';
 $string['requirelowestterms'] = 'Require lowest terms';