Cámbiale las pilas al mando y baja el volumen, que quiero escuchar el diálogo.

Because it is an affirmative command with a pronoun attached.

• cambia = change for tú
• le = to it / for it

In affirmative commands, object pronouns are attached to the end of the verb:

• cambia
  • le → cámbiale

The accent mark is added to keep the original stress of cambia. Without the accent, cambiale would be stressed differently.

Le refers to al mando.

So the structure is basically:

• Cámbiale las pilas al mando
• Change the batteries in the remote

Spanish often uses an indirect object pronoun even when the noun is also stated explicitly. This is very common and very natural.

You will see the same pattern in sentences like:

• Ponle agua a la planta
• Cámbiale el aceite al coche

So le does not mean a person here; it can also refer to a thing.

Because this uses a very common Spanish pattern:

• cambiarle algo a algo/alguien

Literally, it is like change something to something or for something, but in natural English we say change/replace something in something.

So:

• Cámbiale las pilas al mando = replace the batteries in the remote

A learner might expect:

• Cambia las pilas del mando

That version is also possible and natural. The sentence you have is just using a different, very idiomatic structure.

In Spain, mando usually means remote control, especially in everyday speech.

It is short for:

• mando a distancia

So in Spain, people often say simply:

• el mando

In many parts of Latin America, control remoto is more common.

Because a remote control usually takes more than one battery, so the plural sounds natural.

In Spain, pila is the normal everyday word for a small battery such as AA or AAA batteries.

So:

• las pilas = the batteries

If a device used only one battery, then la pila would be possible.

Yes. Baja is the tú imperative of bajar.

Here it means:

• turn down the volume
• lower the volume

This is the normal Spanish way to express that idea:

• sube el volumen = turn the volume up
• baja el volumen = turn the volume down

So this sentence contains two informal commands:

• Cámbiale...
• baja...

Because it uses tú commands.

• cámbiale
• baja

If you were speaking formally to usted, you would say:

• Cámbiele las pilas al mando y baje el volumen, que quiero escuchar el diálogo.

So this sentence is clearly addressed to someone the speaker is on informal terms with.

Here que means because or since.

This is a very common colloquial Spanish structure:

• Haz silencio, que estoy trabajando
• Cierra la ventana, que hace frío

So:

• baja el volumen, que quiero escuchar el diálogo
• turn down the volume, because I want to hear the dialogue

This que is very natural in speech and informal writing.

Because the part after que gives the reason for the request.

The comma helps separate:

• the commands: Cámbiale las pilas al mando y baja el volumen
• the reason: que quiero escuchar el diálogo

With this causal que, the comma is very normal in writing.

Because escuchar usually means to listen to or to listen carefully, while oír means to hear.

Here the speaker wants to pay attention to the dialogue, not just physically hear sound, so escuchar fits better.

Very roughly:

• oír = hear
• escuchar = listen

In real life, the distinction is not always absolute, but this sentence uses the more natural choice.

Spanish often uses the definite article where English might not.

Here el diálogo refers to a specific dialogue already understood from the situation, for example the dialogue in a film, series, or program they are watching.

So el does not sound extra in Spanish; it sounds normal.

It can be literal, as it is here, but there is also an idiomatic expression:

• cambiarle las pilas a alguien

Depending on context, that can mean something like:

• to give someone a wake-up call
• to make someone get moving
• to firmly straighten someone out

But in your sentence it is clearly literal, because the object is al mando, the remote control.