avoid loading audio twice
avoid loading audio twice

fixes issue #125

adrianoesch/jsPsych,Scienthusiast/jsPsych,wolfgangwalther/jsPsych,jspsych/jsPsych,erikasf/jsPsych,jspsych/jsPsych,msrresearch/jsPsych,wolfgangwalther/jsPsych,jspsych/jsPsych,jodeleeuw/jsPsych,adrianoesch/jsPsych,shamrt/jsPsych,Scienthusiast/jsPsych,andyh616/jsPsych,vergenzt/jsPsych,andyh616/jsPsych,erikasf/jsPsych,vergenzt/jsPsych,jspsych/jsPsych,shamrt/jsPsych