Elgato Game Capture HD software includes the ability to record Live Commentary, using a microphone that is connected to your computer.
The interface for Elgato Game Capture HD 2.5.2 for macOS is shown below, but the interface for Elgato Game Capture HD 3.5 for Windows is extremely similar.
Live Commentary - Audio Input
Elgato Game Capture HD now has a Live Commentary section. When you first start the software, Live Commentary is not yet setup.
The Live Commentary section will automatically let you choose between any microphones or headsets that are connected to your Mac or PC.
Live Commentary - Start Speaking
Once you have selected your Audio Input, you will need to turn on Live Commentary, before you start speaking.
Use the Live Commentary button to turn your Audio Input on, or off.
When Live Commentary is on, the Commentary button will turn bright blue.
The Level gauge will also start to animate, with green representing a good level of audio. Your loudest vocal level should probably be somewhere between the middle and the right edge of the green section. If the Level gauge goes beyond that, consider lowering the dial, or else your Live Commentary may become distorted.
Finally, you can tell Elgato Game Capture HD to Automatically reduce game sound. That will lower the gameplay volume while you are speaking, so you can be more easily understood.
The Threshold slider lets you set what audio level your voice should reach before Automatically reduce game sound is activated.
The Attenuation slider lets you set how much to Automatically reduce game sound when you are speaking.
The game sound is reduced by the amount set in Attenuation if the microphone sound reaches the level set in Threshold.
You should always set up your microphone level first, by using the dial control. If you can't really hear you voice, turn the volume up. If your voice is very loud and distorted, turn the volume down.
Then, turn on Automatically reduce game sound. The blue light next to the Automatically reduce game sound checkbox indicates when your voice is loud enough to trigger the effect.
Increase the Threshold if the blue light is always on, even if you're not speaking.
Decrease the Threshold if the blue light doesn't turn on when you speak.
If the game sound gets reduced too much when you are speaking, reduce the Attenuation level.
If you make a recording, that Live Commentary will permanently be in your gameplay, mixed in with the other audio.
If you are using Live Streaming, then your Live Commentary will be sent along with the rest of the video and audio, so anyone who visits your stream can hear you speak while you play.