I øvelokalet står der en højttaler ved vinduet, så alle kan høre melodien godt.

This is because Danish main clauses follow the V2 rule: the finite verb must come in the second position.

Here, I øvelokalet is moved to the front for emphasis or scene-setting, so it takes the first slot. That forces står into the second slot:

I øvelokalet | står | der | en højttaler ...

A more neutral version would be:

Der står en højttaler i øvelokalet ...

Both are correct, but the original starts by focusing on the location.

Here der is an existential dummy word, similar to English there in there is or there stands.

So:

Der står en højttaler = There is a speaker

This der does not mean there as a location. The location is already given by I øvelokalet and later ved vinduet.

Danish often uses der when introducing the existence of something:

• Der er en bog på bordet.
• Der står en højttaler ved vinduet.

Danish often uses position verbs where English would simply use is.
So instead of just saying something is somewhere, Danish may say it stands, lies, hangs, and so on.

Here, står suggests that the speaker is in an upright position:

• Der står en højttaler ved vinduet. = There is / stands a speaker by the window.

If you said Der er en højttaler ved vinduet, that would also be grammatical, but it is more neutral.
Står gives a slightly more physical, visual sense of how the object is placed.

This is the difference between definite and indefinite nouns in Danish.

• en højttaler = a speaker → indefinite
• øvelokalet = the rehearsal room → definite
• melodien = the melody → definite

In Danish, the definite article is usually added as an ending:

• et øvelokale → øvelokalet
• en melodi → melodien

So en højttaler is introducing a speaker as new information, while øvelokalet and melodien are treated as specific or known.

Because Danish loves compound nouns.

Øvelokalet comes from:

• øve = to practise / rehearse
• lokale = room

So et øvelokale literally means something like a practice room or a rehearsal room.

Then the definite ending is added:

• et øvelokale = a rehearsal room
• øvelokalet = the rehearsal room

Writing compounds as one word is completely normal in Danish.

Because ved means by, at, or next to, while i means in.

So:

• ved vinduet = by the window / next to the window
• i vinduet = in the window

In this sentence, the speaker is positioned near the window, not literally inside it.

This is a very common difference in Danish:

• ved døren = by the door
• ved bordet = at the table
• i rummet = in the room

Here så means so, so that, or with the result that.

It connects the first part of the sentence with the result:

... står der en højttaler ved vinduet, så alle kan høre melodien godt.

In other words: the speaker is by the window, so everyone can hear the melody well.

This is not the så that means then in a sequence, although the same word can have that meaning in other contexts.

Because after så, the clause behaves like a subordinate clause, and Danish subordinate clauses normally use subject + finite verb order.

So:

• alle = subject
• kan = finite verb
• høre = infinitive

That gives:

så alle kan høre melodien godt

This is different from a main clause, where Danish often places the finite verb in second position because of the V2 rule.

Kan is a modal verb, meaning can, and høre is the main verb in the infinitive.

So:

• kan = can
• høre = hear

Together:

kan høre = can hear

This works like English modal verbs:

• alle kan høre melodien = everyone can hear the melody

After modal verbs in Danish, you usually use the infinitive without at:

• kan høre
• vil gå
• skal spise

Because godt is an adverb here, not an adjective.

It describes how people hear the melody:

• høre melodien godt = hear the melody well

The basic adjective is god = good, but the adverbial/neuter form is often godt.

Compare:

• en god melodi = a good melody
• de kan høre melodien godt = they can hear the melody well

So godt does not describe melodien; it describes the act of hearing.

Yes. Danish allows several natural variations, depending on what you want to emphasize.

For example:

• Der står en højttaler ved vinduet i øvelokalet, så alle kan høre melodien godt.
• I øvelokalet er der en højttaler ved vinduet, så alle kan høre melodien godt.

Differences:

• I øvelokalet står der ... emphasizes the location first.
• Der står ... is a more neutral way to introduce the speaker.
• er der is more neutral than står der, because står gives a sense of physical position.

So the original sentence is natural, but not the only possible version.