If you want to delete your Sonico account permanently, first login the account and then click into your email id- (my account). You will find your detailed description. In some versions like Portuguese there is a possibility to you find the link over Basic Account tab. Scroll down till the bottom and there you'll find "delete or cancel the account". Click on that and your account get delete. Sonico is going to ask you the reason that you have chosen to close your account. Just tell anything you want and then click ok. Sonico will give you an warning, but just agree with that.