Again, the computer voice is butchering easy pronunciations. The computer voice pronounced "warm" like "von". I know how to pronounce the German "warm" and the computer voice sounded nothing like the way native speakers pronounce the word. Pleas fix the audio. It is quite awful at times, to be frank.
"Ist der Tee warm?" This one is really obvious to me, like Ist: Is Der: the Tee: tea Warm: warm (obviously) I mean, just to me its blatantly obvious to me. But to other people they could've gotten it wrong and that's the best part about Duolingo, you can just learn something until you get it right. :D
I pronounce "warm" like "waam" or "wahm", without an audible /r/ sound. But this may be regional.
Have a look at the two maps on http://www.atlas-alltagssprache.de/runde-1/f16a-b/ -- especially the lower one ("Karte") -- you will see that quite a few people pronounce that as "Kaate", without an "r" (blue dots). That situation is before /t/ rather than /m/ so it might not be quite comparable.
Basically, "hot" is a higher temperature than "warm". "Warm" is more likely to be a comfortable temperature -- perhaps between 20 and 30 °C.
If you have just brewed tea by pouring boiling water over it, it will be "hot" and not "warm" -- too hot to drink.
From learning German in school many, many years ago, I remembered 'warmes Essen' meant hot food, so translated this as 'is the tea hot?' as in 'is the tea hot enough to drink, and not gone cold?' Next time I will be more literal! Though I can't think when this sentence would be useful in real life, unless you were planning to wash your hair in the tea, perhaps ?