As you know, the SOX act came into force in 2002. This was as a result of major financial/accounting frauds involving Enron, World Com and the famous accounting and audit company Arthur Anderson.
I am not sure what depth you need to carry out the SOX audit. If it is a general SOX audit, examining the application of controls, you don't need special SOX certification, although such certification is obtainable.
You need to seriously consider the value to you in the short to medium term vs. investment by studying and passing a SOX Certification. If you have not done CISA and CGEIT, that would give you the skills and knowledge you would want.
I have attached an ISACA paper relating to the SOX audit and hope it will be of help.