كان نفسي اشتري الكتاب الصوتي.

In Egyptian Arabic, كان نفسي is a very common way to express a wish or desire.

• نفسي literally comes from نفس = self / soul
• But idiomatically نفسي often means I feel like, I want, or I wish
• Adding كان gives it a past or slightly regretful feeling

So كان نفسي can sound like:

• I wanted to...
• I really wished to...
• I wish I could have...
  depending on context

It is more emotional than a plain, neutral I wanted.

Because كان shifts the desire away from the immediate present.

Compare:

• نفسي أشتري الكتاب الصوتي = I want / feel like buying the audiobook
• كان نفسي أشتري الكتاب الصوتي = I wanted to buy the audiobook or I wish I could have bought the audiobook

So كان gives the sentence a sense of:

• a past wish
• an unrealized wish
• or a wistful/regretful tone

In Egyptian Arabic, this is a very natural pattern.

Yes, literally نفسي is myself / my soul / my inner self.

But in Egyptian Arabic, it is very often used idiomatically to talk about desire. The idea is something like:

• my heart wants
• my inner self desires
• I feel like

So نفسي is not being used literally here. It is part of a very common everyday expression of wanting.

For example:

• نفسي أنام = I feel like sleeping / I want to sleep
• نفسي في قهوة = I want coffee

Because نفسي behaves differently depending on what comes after it.

If it is followed by a noun, you often use في:

• نفسي في قهوة = I want coffee
• نفسي في الكتاب الصوتي = I want the audiobook

If it is followed by a verb, you usually do not use في:

• نفسي أشتري = I want to buy
• كان نفسي أشتري = I wanted to buy

Since اشتري / أشتري is a verb, no في is needed.

Arabic does not use an infinitive in the same way English does.

In English, you say:

• I wanted to buy

In Egyptian Arabic, after expressions like نفسي or عايز, you usually use the imperfect verb directly:

• نفسي أشتري
• عايز أشتري
• كان نفسي أشتري

So أشتري here does the job that to buy does in English.

In careful spelling, many people would write أشتري.

In informal Arabic writing, especially in dialect, the initial hamza is often omitted, so you may see:

• اشتري

instead of

• أشتري

This is very common in casual writing and texting.

It is still understood as the verb I buy / buy in context, and it is pronounced roughly like ashtiri in Egyptian Arabic.

Yes, absolutely.

كنت عايز أشتري الكتاب الصوتي is also natural and means something like I wanted to buy the audiobook.

The difference is mainly in tone:

• كنت عايز... = more neutral, straightforward
• كان نفسي... = more emotional, heartfelt, sometimes more regretful

So:

• كنت عايز أشتريه = I wanted to buy it
• كان نفسي أشتريه = I really wished I could buy it / I had wanted to buy it

Because in Arabic, adjectives normally come after the noun they describe.

So:

• الكتاب = the book
• الصوتي = audio / sound-related

Together:

• الكتاب الصوتي = the audio book / the audiobook

This is the normal Arabic word order:

• noun + adjective

not

• adjective + noun

Because Arabic adjectives agree with the nouns they describe in definiteness.

So if the noun is definite:

• الكتاب = the book

then the adjective must also be definite:

• الصوتي = the audio

That is why you get:

• الكتاب الصوتي = the audiobook / the audio book

If the noun were indefinite, the adjective would also be indefinite:

• كتاب صوتي = an audio book

In pronunciation, the ل of ال is assimilated before certain letters called sun letters. The letter ص is one of them.

So although it is written:

• الصوتي

it is pronounced more like:

• es-sōti or is-sōti
  depending on how someone represents Egyptian vowels in Latin letters

So the full phrase sounds roughly like:

• el-ketāb es-sōti

not a careful letter-by-letter al-sawti.

Yes, it is clearly colloquial Arabic, and it fits Egyptian Arabic very well.

Things that make it feel Egyptian/colloquial include:

• كان نفسي as a very everyday spoken expression
• casual dialect spelling like اشتري
• the natural spoken-style phrasing overall

A speaker of other Arabic dialects would probably understand it, but it is not Modern Standard Arabic in style.