I have a contract with an agent that expired on August 15, 2012, but the agency has not contacted me on resigning. Am I allowed to switch agents?

Check the terms of your contract and see what it says about termination. Whether you can switch will depend on whether the agreement automatically terminated or else required some action on you or the agent's part to either terminate it or continue the agreement. If you aren't sure, contact a local entertainment attorney who should be able to let you know if the agreement is terminated.