Stemmeomfanget er stort.

Because Norwegian usually writes compound nouns as a single word.
Stemmeomfanget can be broken down like this:

• stemme = voice
• omfang = range, extent
• -et = the definite ending for a neuter noun

So stemmeomfanget is literally the voice-range, i.e. the vocal range.

The base noun is et stemmeomfang. That tells you it is a neuter noun.

For many neuter nouns, the singular definite form ends in -et:

• et hus → huset
• et stemmeomfang → stemmeomfanget

So stemmeomfanget means the vocal range or the voice range.

The dictionary form is stemmeomfang.

If you look it up, you would normally find it under:

• et stemmeomfang = a vocal range

Then the sentence uses the definite form:

• stemmeomfanget = the vocal range

Because stemmeomfanget is a neuter singular noun.

When an adjective is used with a neuter singular noun, it usually takes -t:

• en stor bil = a big car
• et stort hus = a big house

Here, stemmeomfang is neuter:

• et stemmeomfang
• therefore: stort

So Stemmeomfanget er stort follows normal adjective agreement.

Because stort is used predicatively, not directly before the noun.

Compare:

• et stort stemmeomfang = a large vocal range
• det store stemmeomfanget = the large vocal range

But in your sentence, the adjective comes after er:

• Stemmeomfanget er stort = The vocal range is large

After er, Norwegian uses the normal predicative form, not the definite form store.

Yes. Er is the present tense of å være = to be.

So:

• Stemmeomfanget = the vocal range
• er = is
• stort = large / wide / extensive

Structure-wise, it works very much like English:
The vocal range is large.

Yes. In Norwegian, stor / stort / store often has a broad meaning range, including:

• big
• large
• great
• extensive

In this sentence, stort is best understood as something like:

• large
• wide
• extensive

With stemmeomfang, English would often naturally say The vocal range is wide.

No, not really.

• stemmen = the voice
• stemmeomfanget = the vocal range

So:

• Stemmen er stor would sound odd, because a voice is not usually described as simply big in that way.
• Stemmeomfanget er stort specifically talks about the range of notes the voice can cover.

So the sentence is about range, not the voice itself.

Because Norwegian often puts definiteness on the noun itself instead of using a separate article.

English:

• the vocal range

Norwegian:

• stemmeomfanget

The ending -et already contains the idea of the.

Sometimes Norwegian does use a separate determiner too, but not in a simple sentence like this.

Grammatically, you could form that sentence, but it would usually sound unnatural or too general.

• Stemmeomfanget er stort = the vocal range is large

  • this refers to a specific vocal range already known from context

• Et stemmeomfang er stort = a vocal range is large

  • this sounds incomplete or strange unless you are making a very specific comparison or classification

So in normal use, the definite form is much more natural here.

The basic word order is:

• Subject + verb + adjective

So:

• Stemmeomfanget = subject
• er = verb
• stort = subject complement / adjective

This is the standard pattern for simple Norwegian statements, just like in English:

• The vocal range is large

A rough learner-friendly guide is:

STEM-me-om-fang-et

A few tips:

• stemme has a clear e sound, not like English steam
• ng in omfang is pronounced like the ng in sing
• the main stress in Norwegian compounds is usually on the first part, so the beginning of stemmeomfanget gets the strongest stress

You do not need to pronounce every part equally strongly; the word flows as one compound.

Yes, very much so. This is a good example of a sentence that maps closely to English:

• Stemmeomfanget = The vocal range
• er = is
• stort = large / wide

The biggest differences for an English speaker are:

• Norwegian uses a noun ending instead of a separate the
• the adjective must agree with the noun’s gender, so you get stort

Otherwise, the structure is very straightforward.