We do not have such an example. But, we do have 2 different examples. One of which demonstrate speaker functionality, and the other as mic. If you require both in your project, you need to use these 2 reference and club it for your application.
UsbI2sDmaMode: The device enumerates as a vendor specific USB device with two bulk endpoints EP1 OUT and EP2 OUT. The data received on EP1 OUT is streamed to I2S left channel and data received on EP2 OUT is streamed to I2S right channel.
USBAudioClass: The usage of the FX3 USB APIs to implement a standard USB audio class device. The application shows how class specific USB control requests can be processed at the application level.