Learn Danish: Det lyder som om der er en snakken i køkkenet. - It sounds as if there is chatting in the kitchen.

While en snak means "a chat" or a specific conversation, the verbal noun en snakken emphasizes the ongoing action or continuous activity of talking. It translates better to "a chatting" or "a continuous talking," which fits perfectly when describing background noise.

Why does snakken have the article en in front of it? Doesn't the -en ending already make it definite?

It looks identical, but the -en here is not the definite article suffix. It is a completely different suffix used to turn the verb at snakke into a noun (similar to the "-ing" ending in English). These verbal nouns are almost always common gender (en-words), which is why it takes the indefinite article en to mean "a chatting."

Why do we need both som and om in this sentence?

The fixed phrase som om translates directly to the English "as if" or "as though." If you used som alone, it would mean "like" or "as," and om alone would just mean "if" or "whether." You need them combined to introduce this kind of hypothetical scenario.

Could I leave out der er and just say som om en snakken er i køkkenet?

No, that would sound very unnatural. Just like English uses "there is" to introduce the existence of something, Danish uses the existential der er. You cannot just place an indefinite noun as the direct subject of the verb to be when stating that a general action or thing exists in a location.