"There are two cats sleeping under their car."
Translation:Pod jejich autem spí dvě kočky.
I believe so. "Auto" means something like English "automobile" and "vůz" is more broader like "car"(e.g. railroad car) or "wagon". So if you're talking about automobile you can use either.
Hope that helps! (I am native Czech speaker, but not associated with the creators of Czech Course on DL)