Learn Swedish: Vi hör lätena från våtmarkerna. - We hear the calls from the wetlands.

The base form is the neuter noun ett läte, which means a sound, cry, or call, often from an animal. Because it is an ett-word that ends in a vowel, it takes -n in the indefinite plural (läten) and -na in the definite plural (lätena).

How could we expand this sentence using the formal plural relative pronoun vilka (which)?

To say something like 'the wetlands, in which the birds live,' you would use a preposition followed by vilka because våtmarkerna is plural. The continuation would be: ...våtmarkerna, i vilka fåglarna lever. This is a formal alternative to using där (where).

What are the parts of the compound word våtmarkerna and what dictates its noun gender?

It is built from the adjective våt (wet) and the noun en mark (ground, land). In Swedish, the very last word in a compound noun determines its gender and how it declines. Since mark is an en-word, the plural definite ending is -erna, giving us våtmarkerna.

Could we use lyssnar på instead of hör in this sentence?