Easy enough. I'm presuming you're talking about preview and not exporting. Exporting to include/exclude the sound is obvious enough from teh option availabel w3hen you add to the render queue.

Presumably when you are previewing you are just hitting the play button in the VCR type controls top-right yes? You won; thear any sound from that! Go to the menu and find the RAM preview option. That's the only way to get video and audio for preview in AE. However, if you just want audio then just hit the period (".") button on the numeric keypad.

To not play a specific audio layer then just hit the mute button at the far left of the layer.

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