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Wow i totally forgot about this feature until i read this...I just tried it out and is very awesome!
 
Have you tried the User Guide bookmark in Safari, that Apple sticks in nowadays? Look at the bottom of your main bookmark page.

Otherwise, here's the PDF version. See pages 37-38 for available voice commands:

Call someone in contacts Say “call” or “dial,” then say the name of the person. If the person has more than one phone number, you can add “home” or “mobile,” for example.

Dial a number Say “call” or “dial,” then say the number.

Make a correction Say “wrong,” “not that one,” “not that,” “no,” or “nope.”

Control music playback Say “play,” or “play music.” To pause, say “pause,” or “pause music.” You can also say “next song” or “previous song.”

Play an album, artist, or playlist Say “play,” then say “album,” “artist,” or “playlist” and the name.

Shuffle the current playlist Say “shuffle.”

Find out more about the currently playing song Say “what’s playing,” “what song is this,” “who sings this song,” or “who is this song by.”

Use Genius to play similar songs Say “Genius,” “play more like this,” or “play more
songs like this.”

Cancel Voice Control Say “cancel.”


PS.... I just noticed there's a sticky on the forum with the list as well.
 
Also if you just bring up voice control it will scroll different commands in the background
 
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