jordanfox, talk to your employer and make them aware of the Sonic's illegal intentions. Sonic might send a wage assignment to your employer and they will simply have to disapprove it by not out money from your pay stub.
You need to send a revocation letter to Sonic's mailing address and a letter of 'notice of defense' to your employer. Make sure that you have sent these letters through certified mail with return receipt requested and have everything documented in a file.
Sonic will pretend not to work with your consolidation company at the beginning but after a certain stage, they will agree to it. Ask your consolidation company to keep sending the payments. Sonic may not be willing to accept it and return it back to you. If the matter is taken to the court, the judge will understand that you have been trying to pay the debt and it was Sonic that was not working with you. This will make your case stronger and force Sonic to accept the payments from you.
I guess the matter won't reach so far and Sonic will take a lighter turn. Talk to your employer and let them not deduct the money from your payroll department.