Ich höre das Lied wieder, weil es mich fröhlich macht.

Because weil is a subordinating conjunction in German. It introduces a subordinate clause, and in subordinate clauses the conjugated verb usually goes to the end.

So:

• Main clause: Ich höre das Lied wieder
• Subordinate clause: weil es mich fröhlich macht

Compare:

• Es macht mich fröhlich.
• ..., weil es mich fröhlich macht.

That final-verb pattern is one of the most important things to notice with weil.

Both contain the same elements, but after weil, German normally sends the conjugated verb to the end.

So:

• Normal independent clause: Es macht mich fröhlich.
• After weil: ..., weil es mich fröhlich macht.

This is not a special feature of machen; it is the regular subordinate-clause word order.

Because the verb pattern here is jemanden fröhlich machen = to make someone happy/cheerful.

In this pattern, the person affected is in the accusative case, so:

• es macht mich fröhlich
• das macht ihn fröhlich
• das macht sie fröhlich

So mich is correct because it is the accusative form of ich.

Because Lied is a neuter noun: das Lied.

Also, it is the direct object of hören, so it is in the accusative case. For a neuter noun like das Lied, the nominative and accusative forms are the same:

• Nominative: das Lied
• Accusative: das Lied

So Ich höre das Lied means I hear / I’m listening to the song.

In German, hören can often cover both ideas, depending on context.

Here, Ich höre das Lied wieder is most naturally understood as:

• I’m listening to the song again, or
• I hear the song again, depending on context

When talking about music, hören very often corresponds to English listen to.

German does also have zuhören, but that usually means to listen attentively, especially to a person:

• Ich höre Musik. = I listen to music.
• Ich höre dir zu. = I’m listening to you.

Here wieder means again.

So Ich höre das Lied wieder means I’m listening to the song again.

It can also sometimes mean back in other contexts, but here again is the natural meaning.

Examples:

• Ich komme wieder. = I’m coming back / again.
• Ich lese das Buch wieder. = I’m reading the book again.

German word order is flexible, but some positions sound more natural than others.

In Ich höre das Lied wieder, wieder comes late in the clause, which is very normal for adverbs like again.

You could also hear other versions depending on emphasis, such as:

• Ich höre wieder das Lied.
• Wieder höre ich das Lied.

But these have different emphasis or sound less neutral in everyday use.
The sentence given is a very natural standard phrasing.

Because in German, subordinate clauses are normally separated from the main clause by a comma.

So:

• Ich höre das Lied wieder, weil es mich fröhlich macht.

That comma is not optional in standard written German.

This is a very common pattern:

• Ich bleibe zu Hause, weil ich müde bin.
• Sie lernt Deutsch, weil sie in Berlin arbeitet.

Both words can relate to positive feelings, but they are not exactly the same.

• fröhlich = cheerful, merry, in a good mood
• glücklich = happy, fortunate, deeply happy

In this sentence, fröhlich suggests that the song cheers me up or puts me in a cheerful mood. That often fits music very well.

If you said weil es mich glücklich macht, that would sound stronger and more emotionally weighty: the song makes me happy in a deeper sense.

Yes, froh is possible, and froh machen does exist. But fröhlich machen is often a bit more natural when talking about becoming cheerful or bright in mood.

Very roughly:

• froh = glad
• fröhlich = cheerful

So:

• Es macht mich froh. = It makes me glad.
• Es macht mich fröhlich. = It makes me cheerful.

In the context of a song, fröhlich often feels especially fitting.

In informal spoken German, you may sometimes hear something like:

• ..., weil es macht mich fröhlich.

But this is colloquial and not considered standard in careful written German.

For learners, the safest and most correct rule is:

• weil + verb at the end

So you should learn and use:

• ..., weil es mich fröhlich macht.

Yes, often you can, but the nuance is a little different.

• wieder = again
• noch einmal = once more / one more time

So:

• Ich höre das Lied wieder.
• Ich höre das Lied noch einmal.

Both can work. Wieder is usually the more natural everyday choice here. Noch einmal can sound slightly more explicit, as if you mean one more time specifically.

The structure is:

• Main clause: Ich höre das Lied wieder
• Subordinate clause: weil es mich fröhlich macht

More specifically:

• Ich = subject
• höre = conjugated verb
• das Lied = direct object
• wieder = adverb
• weil = subordinating conjunction
• es = subject of the subordinate clause
• mich = accusative object
• fröhlich = adjective/predicative complement
• macht = verb at the end

So it is a very useful example of how German combines a normal main clause with a weil-clause.