Best practices for creating and uploading a mashup
A mashup is a derivative work. If you created a derivative work based on sounds that you have downloaded or found from other sources online - you would always need that content’s creator’s permission to upload it to your account.
Please also bear in mind that crediting the original creator and mentioning that the track has been used for non-commercial purposes is not sufficient. Since it's based on other creators' tracks, you'll need to make sure that the copyright owners of all the works you included in your mashup are fine with you using them in your upload to SoundCloud, even in their edited form.