A plateia segue a letra no telão.

Plateia means the audience as a group. In Portuguese, it is a singular collective noun, so it takes singular agreement:

• A plateia segue...
• not A plateia seguem...

You could also say o público, which also means the audience.
As pessoas means the people, so it is less specific and would be plural.

Because plateia works like a collective noun. Portuguese often treats these group words as singular:

• A plateia aplaude.
• O público canta.

Even though many people are involved, the grammar treats the group as one unit.

Because the subject is a plateia, which is singular. The verb has to match the subject:

• a plateia segue
• as pessoas seguem

So segue is the 3rd person singular form of seguir.

Yes, but here it means something like:

• to follow along
• to keep up with
• to track

In this sentence, seguir a letra means the audience is following the lyrics as they appear on the screen.

In natural English, this is often translated more smoothly as The audience follows along with the lyrics on the big screen rather than the more literal follows the lyrics.

Good question. Letra can mean different things depending on context:

• letter of the alphabet
• handwriting
• song lyrics

In music, a letra usually means the lyrics or the words of the song.

Portuguese often uses the singular where English uses the plural:

• a letra da música = the song lyrics
• literally: the lyric of the song

So in this sentence, a letra is understood as the lyrics.

Because in Portuguese, the lyrics of a song are very commonly treated as a single unit:

• a letra da música

English usually says the lyrics in the plural, but Portuguese normally uses the singular a letra for that idea.

If you say as letras, it usually means the letters of the alphabet, or sometimes different sets of lyrics, depending on context.

No telão means on the big screen.

It is made from:

• em = in / on
• o = the
• em + o = no

And telão is an augmentative form of tela:

• tela = screen
• telão = big screen / large display

So no telão literally means on the big screen.

Portuguese often uses em for both in and on, depending on context.

So:

• na televisão = on TV
• no celular = on the phone
• no telão = on the big screen

English distinguishes in and on more clearly than Portuguese does in many cases.

Yes, it is very common in Brazilian Portuguese. It usually refers to a large screen used in places like:

• concerts
• sports events
• presentations
• karaoke
• outdoor public events

It has a very natural, everyday feel in Brazil.

Yes. Acompanha a letra no telão would sound very natural too.

There is a small nuance:

• seguir = to follow
• acompanhar = to follow along with / accompany / keep up with

In this context, both work well.
Many speakers might even find acompanhar a letra especially natural when talking about reading lyrics while listening to a song.

Yes, Portuguese is fairly flexible here. The original order is completely natural:

• A plateia segue a letra no telão.

But you could also say:

• A plateia segue no telão a letra.
  This is possible, but less natural.
• No telão, a plateia segue a letra.
  This gives extra emphasis to on the big screen.

The most neutral and natural version is the original one.

In practice, it means the audience follows the lyrics by looking at them on the big screen. So no telão is closely connected to the whole action.

A natural interpretation is:

• The audience follows the lyrics on the big screen.

It does not usually mean that the audience itself is physically on the big screen. Context makes that clear.

In this sentence, yes, the articles sound natural and expected:

• A plateia
• a letra

Portuguese uses definite articles more often than English does. Leaving them out here would sound unnatural or incomplete.

For example:

• Plateia segue letra no telão
  This sounds like a headline or very broken Portuguese, not normal everyday speech.

It is partly irregular. In the present tense:

• eu sigo
• você / ele / ela segue
• nós seguimos
• vocês / eles / elas seguem

So in this sentence, segue is the correct form for a plateia.

A learner might expect something like sigue, but that is not correct. The stem changes in some forms.

A few natural translations are:

• The audience follows the lyrics on the big screen.
• The audience follows along with the lyrics on the big screen.
• The audience reads the lyrics on the big screen as they sing along.
  This last one is a bit freer, but it may match the real-life situation well.

The most literal translation is fine, but follows along with the lyrics often sounds more natural in English.