What cable and/or adapter do I need to connect my external speakers to my MacBook Air (original model with fold-down tab with USB, audio out slots)?

The MacBook Air has an audio out jack that fits earphones, but is too narrow for my almost 1/2" round iMac speaker cable. Is there an adapter available for the Air?

Mini DisplayPort to VGA Adapter

Mini DisplayPort to VGA Adapter

Product No Longer Available

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