Я не выбрал песню, не говоря уже о дуэте.

Because выбрал is the perfective past form of выбрать and focuses on the result: the speaker did not end up choosing a song.

• выбрал = chose / managed to choose / ended up choosing
• выбирал = was choosing / tried to choose / used to choose

So:

• Я не выбрал песню = I didn’t choose a song (no result)
• Я не выбирал песню = I wasn’t choosing a song / I didn’t choose a song in a more process-oriented or context-dependent way

In this sentence, the idea is that the speaker failed to choose even the basic thing, so perfective выбрал is the natural choice.

In the past tense, Russian verbs agree with the subject in gender and number.

• выбрал = masculine singular
• выбрала = feminine singular
• выбрало = neuter singular
• выбрали = plural

So Я не выбрал... suggests that the speaker is male.
If the speaker were female, it would be:

Я не выбрала песню, не говоря уже о дуэте.

Because it is the direct object of the verb выбрал.

The noun песня is feminine, and its singular forms are:

• nominative: песня
• accusative: песню

Since choose what? → a song, Russian uses the accusative case:

• Я выбрал песню
• Я не выбрал песню

This is a very common pattern: transitive verbs like choose, see, buy, hear usually take the accusative.

It is an idiomatic expression meaning:

• not to mention
• let alone
• much less

It introduces something that is even less likely, more difficult, or more advanced than what was mentioned first.

So the logic is:

• I didn’t even choose a song,
• let alone a duet.

Literally, it comes from не говоря = not speaking / not even mentioning, but in real usage you should treat не говоря уже о as a fixed phrase.

Because the expression не говоря уже о requires the preposition о, and о takes the prepositional case.

So:

• дуэт = nominative
• о дуэте = prepositional

That is why the sentence says:

не говоря уже о дуэте

This same pattern appears with many nouns:

• не говоря уже о книге
• не говоря уже о машине
• не говоря уже о работе

Because не говоря уже о... is treated as a separate inserted phrase, similar to not to mention... in English.

So Russian normally writes:

Я не выбрал песню, не говоря уже о дуэте.

The comma helps separate the main statement from the added emphatic comment.

Yes, it often could be omitted in Russian if the subject is clear from context:

Не выбрал песню, не говоря уже о дуэте.

Russian often drops subject pronouns when they are obvious.
However, Я may still be included:

• for clarity
• for contrast
• for emphasis
• because it is the start of a sentence

So both are possible, but Я не выбрал... is completely natural.

It is singular.

The basic noun is дуэт = duet.
After о, it becomes о дуэте.

If it were plural, it would be:

• nominative plural: дуэты
• prepositional plural: о дуэтах

So this sentence refers to a duet, not duets in general.

It can mean either, depending on context.

Russian does not have articles like a and the, so песню by itself does not mark definiteness.

So:

• Я не выбрал песню could mean I didn’t choose a song
• or I didn’t choose the song

Usually context tells you which one is meant. In many cases, English uses a when translating this kind of sentence.

It can definitely be used figuratively.

Literally, it might mean someone is choosing music and says they haven’t chosen a song, much less a duet.

But the structure is also widely used in a broader sense to show escalation:

• I couldn’t do the simpler thing, let alone the harder one.
• I don’t have the basic item, not to mention the extra one.

So the pattern X, не говоря уже о Y is very useful in everyday Russian, both literally and figuratively.

It is neutral and very common. It works in:

• everyday speech
• writing
• conversation
• more formal contexts

It is neither especially slangy nor especially stiff.
English equivalents like not to mention and let alone are a good match in tone.

Word-for-word, maybe, but that usually sounds awkward in English.

A more natural translation is:

• not to mention
• let alone
• much less

So while the literal pieces are:

• не говоря = not speaking / not saying
• уже = already
• о = about

you should not translate it too literally. Learn it as a set expression.