It looks like the client ID is either expired or he has removed the app. To fix it you just need to generate your own from the developer portal on Soundcloud, https://soundcloud.com/you/apps
Then you just need to replace the current client id with they one they generate for you. It looks like there are several occurrences of the client id but they are only in two files so if you just find/replace on those it will take care of it. The two files you will want to update are:
soundcloud-is-gold-functions.php
soundcloud-is-gold.php
In the latest version you just need to replace 9rD2GrGrajkmkw5eYFDp2g with the client id soundcloud provided in the developer portal.
Hopefully he will push an update at some point in the future that will either allow you to set the client ID from the admin or at least update the client id that is in the plugin.
