Learn Hardcore Spanish (Latin America): Me sorprende lo musical que es su voz cuando canta. - I am surprised by how musical their voice is when they sing.

You can say Yo me sorprendo but that means I get surprised (myself) and would usually need a de phrase: Yo me sorprendo de lo musical que es su voz…, which sounds heavier and less natural in everyday speech than Me sorprende….

What exactly does lo do in lo musical que es su voz?

Here lo is the neuter article, not the masculine lo (direct object pronoun). It turns the adjective musical into an abstract idea:

musical = musical (adjective)
lo musical = the musical quality / how musical (it is)

So lo musical que es su voz means roughly:

how musical his/her voice is
the degree to which his/her voice is musical

Important points:

Neuter lo is invariable (never la musical, los musical, etc.).
Structure: lo + adjective + que + verb is a common way to say how + adjective + subject + verb.

Is lo musical que es su voz a fixed structure? How does this pattern work in general?

Yes, this is a very common pattern in Spanish:

lo + [adjective/adverb] + que + [verb]

It expresses how [adj] something is or how [adv] someone does something:

No sabes lo cansado que estoy.
You have no idea how tired I am.
Me impresionó lo rápido que hablaba.
I was impressed by how fast he/she spoke.
Se nota lo bien que se llevan.
You can tell how well they get along.

In your sentence:

lo musical → the musical quality
que es su voz → that his/her voice has
Together: lo musical que es su voz → how musical his/her voice is

This whole chunk is one noun-like clause that acts as the subject of sorprende.

Why is it es (indicative) and not sea (subjunctive) after que?

Even though sorprender often triggers the subjunctive, this structure is different.

We use subjunctive with a full noun clause after que when there are two different subjects:

Me sorprende que su voz sea tan musical.
(It surprises me that his/her voice is so musical.)

But in lo musical que es su voz:

lo musical que es su voz is a set exclamative structure:
lo + adj + que + [verb in indicative]
The verb inside (es) is normally indicative, because it’s just stating a fact whose degree is being emphasized.

So:

Me sorprende que su voz sea tan musical. → normal noun clause → subjunctive sea
Me sorprende lo musical que es su voz. → “how + adj” structure → indicative es

Both are correct; they just use different constructions.

Why is cuando canta in the indicative and not cuando cante?

Cuando + verb can use indicative or subjunctive, depending on meaning.

Here we have indicative (canta) because:

It talks about something that actually happens habitually:
- when he/she sings (in general, whenever this happens)

Use indicative for:

Habitual actions: Siempre me llama cuando llega. (He always calls me when he arrives.)
Past events: Me llamó cuando llegó.

Use subjunctive when the action is future/uncertain in time:

Te llamaré cuando llegue. (I’ll call you when I arrive.)
Cuando cante, vas a llorar. (When he/she sings (in the future), you’re going to cry.)

In your sentence, it’s a general fact about the person: cuando canta = whenever he/she sings → indicative.

Who is the subject of canta if there is no pronoun?

The subject is understood from context. Likely:

The subject of canta = the owner of su voz.

Spanish normally drops subject pronouns when the verb ending makes the person clear:

canta → 3rd person singular: he / she / usted / it

Without more context, it could be:

when he sings
when she sings
when you (formal) sing

The connection su voz … cuando canta strongly suggests: when that same person sings.

Can su here mean his, her, your, or their? How do you know?

Yes, su is ambiguous in Spanish. It can mean:

his voice
her voice
your (formal, singular) voice
your (plural, in many countries) voice
their voice

Which one it is depends entirely on context.

If you need to be explicit, you can replace su voz with:

la voz de él (his voice)
la voz de ella (her voice)
la voz de usted (your voice, formal)
la voz de ellos / de ellas (their voice)

In careful speech/writing, people often use de + pronoun/noun to avoid ambiguity.

Could I say Me sorprende lo musical de su voz cuando canta instead? Is it the same?

You can say:

Me sorprende lo musical de su voz cuando canta.

It’s understandable and not wrong, but it’s a bit different:

lo musical que es su voz → emphasizes how musical it is (degree)
lo musical de su voz → focuses more on the musical quality of the voice as a feature

So:

Me sorprende lo musical que es su voz cuando canta.
= I’m surprised at how musical his/her voice is when he/she sings. (emphasis on degree)
Me sorprende lo musical de su voz cuando canta.
= I’m surprised by the musical quality of his/her voice when he/she sings. (emphasis on the trait)

The first (your original) is more idiomatic and emotionally expressive.

Can I say Me sorprende cuán musical es su voz or qué tan musical es su voz instead of lo musical que es su voz?

Yes, there are a couple of alternatives:

Me sorprende cuán musical es su voz cuando canta.
- cuán + adjective = how + adjective
- This is correct and sounds a bit formal or literary in many places.
Me sorprende qué tan musical es su voz cuando canta.
- qué tan + adjective = how + adjective, very common in Mexico and some other regions.
- In other regions, this may sound more colloquial or strongly regional.
Me sorprende lo musical que es su voz cuando canta.
- Very neutral and common across Latin America and Spain.
- Probably the safest, most natural-sounding option.

So all three are understandable; lo musical que is the most universally natural.

Is musical agreeing with voz? Why doesn’t it change form?

Yes, musical describes voz, which is feminine (la voz). However:

Many adjectives that end in -al are invariable in gender:
- un sonido musical, una voz musical
- un estilo formal, una reunión formal

They only change for number:

una voz musical → unas voces musicales

So there is agreement in number (singular), but not in gender, because the adjective’s form is the same for masculine and feminine.

Can I move the parts of the sentence around? For example, put cuando canta at the beginning?

Yes, Spanish word order is quite flexible. These are all natural, with slight differences in emphasis:

Me sorprende lo musical que es su voz cuando canta.
(Neutral order; focus on what surprises you.)
Cuando canta, me sorprende lo musical que es su voz.
(You first set the time/situation, then say what happens.)
Lo musical que es su voz cuando canta me sorprende.
(More marked; emphasizes lo musical que es su voz…)

They all keep the same basic meaning. The original is probably the most typical everyday ordering.