Musikken er festlig.

In Norwegian, definiteness is often built into the noun itself with an ending.
So musikken means the music:

• musikk = music
• musikken = the music

That -en ending is doing the job that the does in English.

Musikk usually refers to music in a general sense, while musikken refers to specific music.

So the difference is roughly:

• Musikk er viktig. = Music is important.
• Musikken er festlig. = The music is festive.

In your sentence, the speaker is talking about particular music, not music in general.

-en is the definite singular ending for many common-gender nouns in Bokmål.

So:

• musikken = the music

Even though musikk is often used as an uncountable noun, Norwegian still uses the definite ending when talking about specific music.

Er is the present tense of å være, which means to be.

So er can mean:

• am
• are
• is

Norwegian uses the same present-tense form for all persons:

• jeg er
• du er
• han er
• musikken er

Because this sentence is using festlig as a predicate adjective, just like in English:

• The music is festive.

In Norwegian, adjectives often come after er when they describe the subject of the sentence:

• Huset er stort. = The house is big.
• Maten er god. = The food is good.
• Musikken er festlig. = The music is festive.

If the adjective came before the noun, it would form a noun phrase instead:

• festlig musikk = festive music

Here, festlig agrees with a singular subject.

After er, adjectives still agree with the noun, mainly in number and sometimes gender. In this sentence, the subject is singular, so festlig is correct.

Compare:

• Musikken er festlig. = The music is festive.
• Sangene er festlige. = The songs are festive.

So festlige would normally be used with a plural subject.

Festlig usually means something like:

• festive
• cheerful
• lively
• fun
• party-like

With music, it suggests that the music creates a happy or celebratory atmosphere. Depending on context, festlig can sound slightly broader than just festive in English.

This sentence has very normal Norwegian word order:

• Musikken = subject
• er = verb
• festlig = complement

So it follows the basic pattern:

subject + verb + description

Also, Norwegian main clauses follow the V2 rule, which means the finite verb usually comes in the second position. In this sentence, the subject comes first, so the verb naturally comes next.

A simple English-friendly guide would be:

moo-SIK-ken er FEST-lig

A few notes:

• The stress in musikken falls on the second syllable: mu-SIK-ken
• The u in Norwegian is not exactly the same as English oo
• The r in er varies by dialect
• festlig is usually pronounced with a clear stl cluster, though actual pronunciation can vary a bit depending on dialect and speaking speed

They are related, but they are not the same kind of structure.

• festlig musikk = festive music
  
  • this is a noun phrase
• musikken er festlig = the music is festive
  
  • this is a full sentence

So one names a type of music, while the other makes a statement about the music.