Learn Icelandic: Stelpurnar sofa stundum hjá vinum. - The girls sometimes sleep at friends' houses.

It comes after the preposition hjá (at/with), which always forces the noun into the dative case. In Icelandic, the dative plural ending for almost all bare (indefinite) nouns is -um. So, the basic plural vinir (friends) becomes vinum.

Where is the word for "houses" in this sentence?

There isn't one! Using the preposition hjá followed by a person is the standard way to say "at [someone's] house" or "at [someone's] place". Literally, hjá vinum just means "by friends" or "with friends".

The word stundum (sometimes) also ends in -um. Is it a dative plural noun too?

Historically, yes! It was originally the dative plural of the noun stund (a moment or while). However, in modern Icelandic it acts as a fixed adverb meaning "sometimes". You just treat it as a single vocabulary word and never change its ending.

Why is the word order sofa stundum instead of "stundum sofa" like the English "sometimes sleep"?

Icelandic has a strict "verb-second" rule in main clauses. The verb sofa (sleep) must take the second slot in the sentence, right after the subject Stelpurnar (The girls). Because of this, the adverb stundum (sometimes) is pushed to the spot right after the verb.