The Ultimate Screenplay Format Reference

Slow Motion

If you read the other pages, this goes without saying: Don’t use slow motion just because you think it’s cool. Use it only if it is important to the plot. That said - if you have a story related reason then the Screenwriter’s Bible suggests a format similar to the montage:
 

INT. JANE’S OFFICE

Jane presses a button. The film progresses in slow motion:

TIME LAPSE

-- Richard pulls his gun from the holster, points it at Jake

-- Richard is hit in the chest, a red spot forms on his shirt

INT. CRIME SCENE

Jane and Frank at the crime scene. Frank takes the position of
Richard when he was shot. ...
 

You get the idea:

TIME LAPSE

-- <first shot>

-- <second shot>

-- etc.

Here the time lapse sequence is terminated by the next master scene heading. If you want the action to continue within the same scene after the time lapse, then a BACK TO SCENE should do the job.

Music

Refer to music only if it is relevant for the story. If it is, this is a way to do it:
 

Jake awakes to a HEAVY-METAL RIFF and instantly presses both
hands on his ears.
 

Because music is sound, you could also describe it as such - putting the sound in caps, if you want:
 

The radio BLASTS heavy-metal music into the quiet neighborhood.
A cat runs for shelter.
 

As explained in the section on sounds, the caps in ‘BLASTS’ are optional.

Do NOT indicate specific songs unless you own the rights. If you know how much the rights for ‘Sympathy for the Devil’ cost the makers of Fallen (imdb), a 1998 movie written by Nicholas Kazan then don’t hesitate to email. They certainly came at a considerable fee.

Music Lyrics

The same as for music goes for music lyrics: If you don’t own the rights, then don’t include them in your script.

In all other cases, for example if the song is in the public domain (‘Jingle Bells’, ...), a character sings nonsense or you create the lyrics yourself (in which case you own the rights) then these example from The Screenwriter’s Bible suggests how to do it:
 

                   McKAY
         “Well, you take the high road
         And I’ll take the low road,
         And I’ll be in Scotland before you.”
 

Alternatively you can use slashes instead of the line breaks - may save space occasionally:
 

                   McKAY
         “Well, you take the high road/and
         I’ll take the low road/and I’ll
         be in Scotland before you.”
 

Movie Clips

It all boils down to the rights - do you own them or not. If you don’t own the rights then don’t make your script dependent on a specific movie or movie clip.

However, you can briefly refer to other movies. The Screenwriter’s Bible quotes ‘Sleepless in Seattle’ (imdb link) (story by Jeff Arch, screenplay by Nora Ephron), which has this passage:
 

Annie is watching "An Affair to Remember" on television,
tears pouring down her face.  Cary Grant is saying: "Are
you in love with him?" Deborah Kerr replies: "I'm not now."
 

On the other hand, The Screenwriter’s Bible advises not write ”He turned on the TV and the sinking scene from TITANIC was showing.”

Balance these two suggestions in your mind and heart.

When in doubt, don’t write it.