Since the users can record the sound on your site I think this is a good candidate for using mechanical turk. You can review the work users submit and accept (t

Since the users can record the sound on your site I think this is a good candidate for using mechanical turk. You can review the work users submit and accept (thus paying them) or reject (deny them pay). Obviously if you deny a lot of people, you will have a reputation of not paying people, but I think if you give people very clear guidelines and review what people submit it will work. You can also add filters like only allow people with approval ratings of greater than 90% to try and ensure you get good workers.