The button is in near the topic's header. As a contributor, I can also check sentences in the Incubator and to both normal and slow pronunciation there (does not happen with non-existing sentences, though—and we do have the issue of users encountering "ghost" sentences, the ones we deleted months ago)
To me, this English sentence doesn't make sense unless you imagine a very unusual situation. "This bus" or "that bus" means you are looking at it, but "not seen yet" means you haven't seen it. I don't know if the Russian makes sense, but the correct English would be "I haven't seen that bus before".