click on advanced in the top right> then click on the speaker icon in the bottom right uncheck the follow audio icon, (orange and green arrow) like in the screen capture down the post, for the audio input that you want active all the time. Also make sure that the green speaker is checked on this source. Second uncheck the same button (the follow green and orange button) for the audio source you dont want to use.

If you want to play audio with a video the audio should automatically start when the input turns live. But remember cause you have deselect the follow audio on the camera to manally turn the audio on the camera down by moving the slider down or depressing the speaker icon (or audio icon under the source).