A is typically a URL that directs you to a web-based proxy server. You visit the proxy site, enter "soundcloud.com" into a text box, and the proxy fetches the page for you. To the school or office Wi-Fi firewall, it looks like you are visiting the proxy, not SoundCloud.
Sometimes the block is based on the "rich media" of the standard player. Try forcing the mobile version. soundcloud unblocked 66 link