The instructions for canceling are here. On the same page you can also find support@audio.com. (But with a “published” email address I’m not sure how they can sort through the spam.)
If you can’t get a refund and you feel you were cheated, try contacting your credit card company.
Personally, I’d be very careful about giving my credit card number for a “free trial”. 
I don’t think I’ve ever done it.