Мы не нашли песню, не говоря уже о минусовке.

Because it is the direct object of the verb нашли.

The dictionary form is песня = song, but after найти / нашли = to find / found, the thing found goes into the accusative case.

So:

• песня = nominative singular
• песню = accusative singular

This is a very common pattern in Russian:

• Я слушаю музыку = I listen to music
• Мы нашли песню = We found a song

Since песня is feminine, its accusative singular changes from -я to -ю.

It is an idiomatic expression meaning:

• let alone
• not to mention
• much less

So the sentence means that they did not even find the song, and finding the backing track was even more unlikely.

Literally, не говоря уже о is something like not even speaking about or not to speak of, but in normal English the best translation is usually let alone or not to mention.

Examples:

• У нас нет времени на прогулку, не говоря уже о поездке.
  We don’t have time for a walk, let alone a trip.

• Он не может купить машину, не говоря уже о доме.
  He can’t buy a car, let alone a house.

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

So:

• минусовка = dictionary form
• о минусовке = about the backing track

That is why you get минусовке.

A similar pattern:

• Я думаю о книге = I’m thinking about the book
• Мы говорим о музыке = We are talking about music
• Не говоря уже о минусовке = Let alone the backing track

So the case here is determined by the preposition о inside the fixed expression.

Минусовка is a music-related word meaning a backing track, instrumental version, or karaoke-style accompaniment without the main vocal.

It comes from the idea of the vocal being removed or subtracted. In everyday use, it often means the version someone can sing over.

Depending on context, it could be translated as:

• backing track
• instrumental track
• karaoke track

So in this sentence, the idea is:

We couldn’t even find the song itself, let alone an instrumental/backing version of it.

Because найти and искать mean different things.

• искать = to look for, to search for
• найти = to find, to succeed in locating

So:

• Мы не искали песню = We didn’t look for the song
• Мы не нашли песню = We didn’t find the song

In your sentence, the point is about the result: they tried or were looking, but in the end they did not find it.

This is also why нашли is the perfect choice with не говоря уже о: the sentence compares results, from one thing not found to an even less likely thing.

Нашли is:

• past tense
• plural
• from the verb найти
• perfective aspect

Why plural? Because the subject is мы = we.

So:

• я нашёл / нашла = I found
• ты нашёл / нашла = you found
• мы нашли = we found

The perfective aspect is important too. Найти is perfective, so нашли focuses on a completed result:

• искали = were looking / looked for
• нашли = found

With negation, не нашли means did not manage to find.

Because не говоря уже о... functions as a set expression that adds a comment to the main clause. In writing, it is usually separated by a comma.

Main clause:

• Мы не нашли песню

Added phrase:

• не говоря уже о минусовке

So the comma marks the second part as an added idiomatic phrase, similar to how English separates let alone... or not to mention... from the main statement.

This punctuation is standard and very natural.

Yes, Russian word order is flexible, although the neutral version here is very natural:

• Мы не нашли песню, не говоря уже о минусовке.

You could also hear or see:

• Песню мы не нашли, не говоря уже о минусовке.
• Мы песню не нашли, не говоря уже о минусовке.

These all mean basically the same thing, but the emphasis changes slightly:

• Мы не нашли песню... = neutral
• Песню мы не нашли... = emphasizes the song
• Мы песню не нашли... = also gives a bit more emphasis to песню

So the original sentence is a good standard model.

Russian often can omit personal pronouns when the subject is clear from the verb form or context. So yes, you could say:

• Не нашли песню, не говоря уже о минусовке.

That can sound natural if the subject is already understood.

However, мы is perfectly normal too, and including it can make the sentence clearer or slightly more explicit.

So both are possible:

• Мы не нашли песню... = explicit we
• Не нашли песню... = we is understood from context

Не говоря уже о is generally neutral and very common. You can use it in conversation, writing, and many everyday situations.

The more informal element in the sentence is actually минусовка, because that word belongs to music-related everyday speech and jargon.

So the overall sentence sounds natural and conversational, but not strange or sloppy.

You could use не говоря уже о in many registers:

• everyday speech
• journalism
• essays
• casual written Russian

It is a very useful phrase to learn because native speakers use it often.