Both work. Skipping the den/det makes it sound more like a title/fixed expression:
Tredje mannen - The third man
Sjätte sinnet - The sixth sense
It appears so. But I would also like to hear a native explain why in this case. There are some adjectives such as fel that never take the article but not sure if that is the case here.
The article can often be left out with numerals, see HelenCarlsson:s examples above. And adjectives like sista 'last', nästa 'next' etc work the same. It's like saying that something is the first, third, next, last or whatever, in itself conveys enough information to show that it is definite.
Just to check, can this mean both, votes from the opposing party and voices from the beyond?
I listened at normal speed a number of times and it sounded like "rösten är från andra sidan"
To me, it sounds right though. Maybe it's a case of "tuning" your ears to hear the difference between N and RN?