Baja el volumen del altavoz, por favor.

Baja is the affirmative imperative (a command) for tú (informal you) from the verb bajar (to lower / to bring down). It’s telling someone: Lower the volume.

Bajar can mean both to go down (intransitive) and to lower something (transitive). Here it’s transitive because it has a direct object: el volumen. So it means lower/turn down.

As written, it’s informal because baja is the tú command.
Formal (usted) would be: Baje el volumen del altavoz, por favor.
Plural forms:

• Bajen el volumen... (ustedes / plural you, common in Latin America)
• Bajad... (vosotros, mainly Spain)

With voseo, the command is typically: Bajá el volumen del altavoz, por favor.
Spelling often includes the accent: bajá.

Del is the mandatory contraction of de + el.
So del altavoz = de el altavoz (but Spanish normally contracts it to del).

Yes. Altavoz commonly means a speaker (a loudspeaker). Depending on the country, you may also hear parlante or bocina for a speaker, especially in everyday Latin American usage.

Yes, volumen is typically masculine: el volumen.
So you say sube el volumen (turn it up) / baja el volumen (turn it down).

Yes, very common alternatives include:

• Bájale al volumen, por favor. (very conversational; uses an indirect object pronoun)
• Baja el sonido, por favor. (lower the sound)
• Baja la música, por favor. (lower the music)

• Baja el volumen...: straightforward command with a direct object (el volumen).
• Bájale el volumen...: adds le (often meaning to it/to him/to her, depending on context). In practice it often feels like turn it down.
• Bájale al volumen...: very common in speech; it’s like turn it down (the volume), with al volumen behaving like the target of the action.

All can be natural; which one people prefer varies by region and style.

You can place por favor in several spots without changing the core meaning:

• Baja el volumen del altavoz, por favor.
• Por favor, baja el volumen del altavoz.
• Baja, por favor, el volumen del altavoz. (less common, but possible)

Placement mostly changes tone/emphasis, not meaning.

To say Don’t turn down the volume (tú):

• No bajes el volumen del altavoz.

To say Don’t turn it up (tú):

• No subas el volumen del altavoz.

Formal (usted):

• No baje... / No suba...