Baja el volumen, por favor, que mi hija ya está dormida.

Baja is the affirmative tú command of bajar.

• bajar = to lower / to turn down
• bajas = you lower / you are lowering
• baja = lower! / turn down!

So Baja el volumen means Turn the volume down when speaking to one person informally.

A quick pattern:

• tú hablas → Habla
• tú comes → Come
• tú bajas → Baja

The affirmative tú command usually looks like the él/ella/usted present-tense form.

Yes. Baja is informal singular, used with tú.

Other possibilities are:

• Baja el volumen = informal, to one person
• Baje el volumen = formal, to one person
• Bajad el volumen = informal, to more than one person in Spain
• Bajen el volumen = formal plural, or general plural in many Spanish-speaking countries

Since you asked about Spanish from Spain, bajad is especially worth noticing because Spain uses vosotros for informal plural.

Because volumen is a masculine noun in Spanish, so it takes el.

• el volumen = the volume

Even though English learners may focus on the idea of volume as an abstract thing, Spanish still uses the definite article here. This is very normal in commands:

• Cierra la puerta = Close the door
• Apaga la luz = Turn off the light
• Baja el volumen = Turn down the volume

Here que means something like because or since.

So:

• Baja el volumen, por favor, que mi hija ya está dormida.

is like saying:

• Please turn the volume down, because my daughter is already asleep.

This use of que is very common in everyday Spanish, especially in speech. It often introduces the reason for what was just said.

A slightly more explicit version would be:

• ... porque mi hija ya está dormida.

Both are correct, but que often sounds more natural and conversational here.

Both are possible, but they are not exactly the same.

• duerme = she is sleeping
• está dormida = she is asleep

In this sentence, está dormida focuses on her state: she is already asleep. That is why lowering the volume matters now.

So the sentence is not just describing the action of sleeping; it is emphasizing the result or condition: she’s asleep already.

Ya here means already.

• mi hija está dormida = my daughter is asleep
• mi hija ya está dormida = my daughter is already asleep

It adds the idea that this has already happened by now, which makes the request sound more urgent or justified. In natural English, you might or might not translate it directly every time, but in Spanish it helps the sentence feel more complete and natural.

Because dormida agrees with mi hija, which is feminine singular.

• dormido = asleep, masculine singular
• dormida = asleep, feminine singular
• dormidos = masculine plural / mixed plural
• dormidas = feminine plural

Examples:

• Mi hijo está dormido = My son is asleep
• Mi hija está dormida = My daughter is asleep

Here dormido/dormida is functioning like an adjective.

Spanish uses estar for many temporary states or conditions, and being asleep is one of them.

So:

• está dormida = she is asleep

You would not normally say es dormida here.

A useful contrast:

• ser = identity, essential characteristics, classification
• estar = state, condition, location

Since asleep is a temporary condition, Spanish uses estar.

It is direct, but not rude. The phrase por favor makes it polite.

• Baja el volumen by itself can sound fairly strong, depending on tone.
• Baja el volumen, por favor sounds more courteous.

In everyday family or home situations, this is very natural. If you wanted to sound softer, you could say things like:

• ¿Puedes bajar el volumen, por favor? = Can you turn the volume down, please?
• Baja un poco el volumen, por favor = Turn the volume down a bit, please

So the original sentence is perfectly normal and polite enough in many contexts.

Because por favor is inserted parenthetically in the middle of the sentence.

So:

• Baja el volumen, por favor, que mi hija ya está dormida.

The commas show a little pause, like in English:

• Turn the volume down, please, because my daughter is already asleep.

You could also place por favor at the beginning or end:

• Por favor, baja el volumen...
• Baja el volumen, por favor.

All are correct.

Yes. You could say:

• Baja el volumen, por favor, porque mi hija ya está dormida.

That is fully correct and very clear.

The difference is mostly one of style:

• que sounds more conversational and immediate
• porque sounds a little more explicit and neutral

In this kind of everyday spoken sentence, que is extremely natural.

Yes. In this context, bajar el volumen is the normal way to say turn the volume down.

Literally, bajar means to lower, but with things like sound, music, TV, radio, etc., it is commonly used the way English uses turn down.

Examples:

• Baja la música = Turn the music down
• Baja la tele = Turn the TV down
• Baja el volumen = Turn the volume down

So this is a very useful everyday expression.