صندوق الرمل فيه رمل القطط.

A common Egyptian pronunciation is:

ṣandūʔ er-raml fīh raml il-ʔoṭaṭ

A few notes:

• صندوق → ṣandūʔ
• الرمل → er-raml because ر is a sun letter, so the ل of ال is absorbed
• فيه → fīh
• القطط → il-ʔoṭaṭ; in Egyptian, ق is often pronounced as a glottal stop ʔ

You may also hear slight vowel differences like el-/il- or er-/ir- depending on the speaker.

Here فيه literally means in it:

• في = in
• ـه = it / him

So the structure is basically:

صندوق الرمل فيه رمل القطط
= The litter box, in it there is cat litter
= more natural English: The litter box has cat litter in it

In Egyptian Arabic, X فيه Y is a very common pattern for saying that something contains something.

It refers back to صندوق because صندوق is masculine.

So:

• فيه = in it masculine
• If the noun were feminine, you would usually say فيها

Example:

• الشنطة فيها فلوس = The bag has money in it

Because Arabic often leaves out the present-tense verb to be.

So instead of saying something like:

• The box is containing cat litter
• or The box has cat litter

Egyptian Arabic commonly uses a location/content structure:

• صندوق الرمل فيه رمل القطط
• literally: The box, in it cat litter

This is totally normal Arabic grammar.

Because صندوق الرمل is an iḍāfa structure, often called a construct phrase.

That means:

• صندوق = box
• الرمل = the sand

Together: box of the sand → sand box / litter box

In an iḍāfa:

• the first noun usually does not take ال
• the second noun can be definite or indefinite
• the whole phrase becomes definite if the second noun is definite

So صندوق الرمل is correct, while الصندوق الرمل would not be the normal structure for this meaning.

Because these are two different noun phrases.

1) صندوق الرمل

This is an iḍāfa:

• صندوق
  • الرمل
• box of the sand
• the whole phrase is definite because الرمل is definite

2) رمل القطط

This is also an iḍāfa:

• رمل
  • القطط
• sand/litter of the cats

In an iḍāfa, the first word normally does not take ال, so رمل القطط is expected.

So the difference is grammatical, not random.

Yes. Literally, it means something like cats’ sand/litter.

That is a very normal way for Arabic to build everyday compound meanings. In context, رمل القطط is understood as cat litter.

Also, رمل can refer not just to literal beach sand, but to the sand-like or granular material used for cats.

So this expression is quite natural.

Because the meaning is generic: litter for cats in general.

Using the plural القطط makes sense for a general category. If you used the singular القطة, it would sound more like:

• the cat’s litter/sand
• meaning one specific cat

So رمل القطط is the natural generic expression.

Yes, literally it could.

By itself, صندوق الرمل can mean sand box in a general sense. Context tells you what kind of box it is.

In this sentence, once you add رمل القطط, it becomes clear that we are talking about a cat litter box.

Depending on the speaker, you might also hear other ways to make that clearer, but this sentence is understandable.

It may feel repetitive in English, but it is perfectly normal here.

The two uses do different jobs:

• صندوق الرمل = the name/type of the box
• رمل القطط = the material inside it

So the sentence works like English:

• The litter box has litter in it

English also repeats the same root idea, and Arabic is doing something similar.

It can do both, depending on context.

• On its own, فيه often means there is
  • فيه ميه = There is water
• After a noun, it can feel more like in it
  • الصندوق فيه ميه = The box has water in it

In this sentence, because صندوق الرمل comes first, learners often understand فيه best as in it, though the idea of there is is also built into the structure.