The command to fade the volume level of one or more audio sources is fade

The parameters are;

• volume=<new volume from 0.0 to 1.0>
• steps=<number of steps for the fade effect>
• interval=<time in millisecs between each step>
• trigger=<number or name of the deck button to trigger when complete> (optional - added in lua script v2.1.19)
• delay=<time in millisecs> (optional)
• <audio source>
• <audio source> (optional)
• <audio source> (optional)

For more info on the optional trigger, see Countdown timers

The optional delay specifies a time (in milliseconds) to wait before running the command.

Example - fade the volume of the Mic and Music audio sources from their current levels to 0.  The fade effect is split into 100 steps with a 50ms interval between each step.