QTCaptureDevice *theDefaultMuxedDevice;
QTCaptureDeviceInput *theDeviceInput;
BOOL success;
NSError *error;

...

// get the default muxed device
theDefaultMuxedDevice = [QTCaptureDevice defaultInputDeviceWithMediaType:QTMediaTypeMuxed];

// open the device
success = [theDefaultMuxedDevice open:&error];
if (YES == success) {
    // get the associated device input
    theDeviceInput = [QTCaptureDeviceInput deviceInputWithDevice:theDefaultMuxedDevice];

    // get the list of owned connections 
    NSArray *ownedConnections = [theDeviceInput connections];

    // disable all the audio connections
    for (QTCaptureConnection *connection in ownedConnections) {
        if ( [[connection mediaType] isEqualToString:QTMediaTypeSound] ) {
            [connection setEnabled:NO];
        }
    }
} else {
    // do something with the error code
}

...