デュアル ヘッドホン用ヘッドホン スプリッター (3.5 mm プラグからデュアル 3.5 mm ジャック) ステレオ オーディオ ケーブル

You can set the sound input device in the Sound utility.
Go through the following:
1. Click Apple icon.
2. Click System Preferences.
3. Click Sound.
4. Click Input tab.
5. Click the input device you would like to use.

You can set the audio output device in the Sound utility.
Go through the following:
1. Click Apple icon.
2. Click System Preferences.
3. Click Sound.
4. Click Output tab.
5. Click the output device you would like to use.

You can change your default recording device in Sound. 
Go through the following:
1. In the bottom right of the Desktop, right-click the speaker icon.
2. Click Recording devices.
Note: A list of all available recording devices should be displayed.
3. Right-click on the desired device for default recording.
4. Click Set as Default Device.