When using XSplit Broadcaster to record videos, you can enable Multitrack Audio to separate your recording's audio into four discrete tracks; Mixed, Microphone, System Sound, and Stream Only. On XSplit Express Video Editor, you'll then be able to choose which audio tracks will be in your final exported video.
To choose the audio track you would like to use with a specific clip, click on the drop-down below the video clip's file path. Alternatively, you can right-click on the clip then select Audio Track from the pop-up menu.
To learn more about enabling multitrack audio on your XSplit Broadcaster recordings, click here.
For XSplit Gamecaster, go to the Settings tab then click the gear icon under Record Settings then enable Create Multitrack Recording on the Custom Record Settings window.