Spoiler: Theories on why
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Toph can listen to the swamp.


Theory 1:

Cloudbending exists. I'm not sure how much of it was Aang and Katara working together and combining the elements, but it seems plausible that airbenders and waterbenders could individually bend clouds.

Lavabending exists. It seems to be a similar combination of earth- and firebending. We've seen earthbenders do it own their own, and we might have seen firebenders as well. Although to be fair, the only firebenders we've seen do it have been Avatars. But that list includes Unnamed Fire Avatar, so it seems plausible that it's a firebending technique as well.

Perhaps there are similar combinations for other elements. Water/Earth could be mudbending, of course. But with this theory, I could also see communicating with plants and the earth as being a joint water-earth technique.

Theory 2: Perhaps it's just a skill available to anyone who's really spiritual. They never actually said she was plantbending like Huu.