Fale em um tom calmo.

Because fale is the imperative form used with você (and also o senhor / a senhora).

• falar = to speak
• fale = speak / speak in this way

In Brazilian Portuguese, affirmative commands for você use the same form as the present subjunctive:

• que você fale
• fale!

So Fale em um tom calmo is a command or instruction meaning Speak in a calm tone.

If you were using tu, the form would usually be fala:

• Fala em um tom calmo.

But in Brazil, usage with tu varies a lot by region, so fale is a very safe form to learn.

Not necessarily.

It matches você, which is very common in Brazil and is often neutral, not especially formal. It can also match o senhor / a senhora, which is more formal.

So the sentence feels more like:

• a neutral instruction
• a polite but direct request
• something you might see in a prompt, script, classroom, or customer-service context

If you want to make it softer, you could say:

• Por favor, fale em um tom calmo.
• Pode falar em um tom calmo?

Portuguese often leaves out the subject pronoun when it is already clear from the verb form or the context.

So instead of saying:

• Você fale em um tom calmo

Portuguese normally just says:

• Fale em um tom calmo

In fact, você fale by itself would usually sound unusual here. For commands, Portuguese typically uses just the imperative verb.

Because falar em um tom... is a natural way to talk about the tone or manner of speaking.

Here, em works like in in English:

• Speak in a calm tone

So:

• em um tom calmo = in a calm tone

You may sometimes see other structures in Portuguese, but falar em um tom calmo is a very good, natural pattern to learn.

Here, tom means tone — the quality or emotional coloring of the voice.

So this is not about:

• color tone
• musical tone only

It is about how the voice sounds while speaking.

Examples:

• um tom calmo = a calm tone
• um tom sério = a serious tone
• um tom agressivo = an aggressive tone

Because calmo describes tom, and tom is a masculine noun.

So the adjective has to agree with it:

• um tom calmo
• um tom sério
• um tom baixo

If the noun were feminine, the adjective would change:

• uma voz calma

So the adjective is matching tom, not the person who is speaking.

Yes. That is correct, but it is slightly different in nuance.

• Fale calmamente = Speak calmly
• Fale em um tom calmo = Speak in a calm tone

The first focuses on the overall manner of speaking.
The second focuses more specifically on the tone of voice.

Both are natural, but em um tom calmo is a bit more specific.

Yes. It sounds natural, especially in situations like:

• giving instructions
• directing an actor or speaker
• asking for a softer tone
• writing a prompt for an AI or assistant

In everyday conversation, Brazilians might also use simpler or more casual alternatives, such as:

• Fala com calma.
• Fale com calma.
• Fala mais calmo.
• Pode falar com mais calma?

These are a bit more conversational.
Fale em um tom calmo sounds clear, correct, and slightly more deliberate.

A rough English-friendly approximation is:

FAH-lee eeng oong tohn KAHW-moo

A few helpful notes:

• fale = FAH-lee
• em um is often reduced in fast speech, so it may sound more connected
• tom has a nasal vowel, so it does not sound exactly like English Tom
• calmo in Brazil often sounds close to KAHW-moo, because the l changes into a w-like sound

So in natural speech, the whole sentence may sound smoother and more connected than the spelling suggests.