The easiest way to bring sound into your encoding computer is typically the microphone input port. This is typically a 1/8" audio jack and is commonly labeled "Line In" in many encoders. You may need an adapter to get your audio system input to feed to the microphone port. In most cases, common adapters can be found at local electronics stores.
If you are plugged into your microphone input port and are still not having the sound work, please double check that you did not plug into the computer's "Headphones" port as the headphone and microphone ports are usually right next to each other. In addition, if you have confirmed the setup is correct, it would be best to check the line you are feeding to the microphone port is actually working by hooking a speaker or some other speaker type device to it to confirm there is sound going through the cable.
If you still aren't able to bring in audio through the microphone input port then you may need to bring it in through your video capture device or through an audio capture device.