After converting a video clip into a Smart Object, the options for Audio and Video (including the mute option) change to Motion options.

Once a clip is converted to a Smart Object in Photoshop, the Audio and Video options change to Motion options.
To access the Audio and Video options after converting the clip to a Smart Object, double click on the Smart Object’s thumbnail in the Layers panel. The contents of the Smart Object will open in its own window. On the Timeline panel, use the triangle in the upper right of the clip to access the Video and Audio options. Click the Audio icon and check Mute Audio. Then, save the open document (the “contents” of the Smart Object). Upon close, the smart object updates in the original document.
Thank you for this post. It was exactly what I needed!
how do you access the audio after you made a smart object?. as you say the control becomes motion! the audio is part of the video. do you have to control before you make it into a smart object? I just want to must some parts that i sniped.
Double click on the smart object thumbnail in the Layer’s panel. It will open in it’s own window where you can then access the audio options for the clip.