"De" and "dem" are pronounced like /dom/ in most dialects as well as in rikssvenska (this is why many Swedes confuse them when writing).
In some dialects in northern Sweden and in Finland (finlandssvenska) you pronounce those two words just like you read them (/dɛ/ and /dɛm/)
Sometimes /dɛ/ and /dɛm/ also appear in formal speech.
Okay, I did the exercise again and it definitely is unvoiced and sounds more like a "y" in English. But Spanish does that buzzing thing too with its y sound so it can sound like a j sometimes. It's just the way the human vocal structures are, I guess. Thank you, Helen! <3