Learn French: Les ordinateurs sont sous la table. - The computers are under the table.

Yes, as a "z" sound! Normally, the 's' in les is silent. But because the next word, ordinateurs, starts with a vowel, the words link together. This is called a liaison. You pronounce the two words together like "lez-ordinateurs".

Do I pronounce the 's' at the end of ordinateurs?

No, the plural 's' at the end of French nouns is almost always silent. When speaking, the only way people know you mean more than one computer is because they hear the plural article les at the beginning!

Why use les here instead of des?

Les is the plural of "the", so you use it when talking about specific things (the computers). Des means "some". If the sentence meant "There are some computers under the table," you would use des instead.

I always mix up sous and sur. Which one is this?

It is a very common mix-up! Sous means "under" and is pronounced with an "oo" sound, like "soup". Sur means "on" and uses the tight-lipped French "u" sound. In this sentence, the computers are under (sous) the table.