Misplaced modifiers?

The first because you heard the snoring not the sound asleep.

Either could do, but both reflect an unnecessary problem. It can be solved by removing the words "sound asleep." The resulting sentence would be: "I heard Denmark snoring on the porch." The reason to delete "sound asleep" is that this is unnecessary and redundant because people do not snore while they are awake.

I prefer the first, but with a comma to separate the two clauses - I heard Denmark snoring, sound asleep on the porch. If you use the second version, again you need a comma, after "porch".

It's a matter of punctuation.
"I heard Denmark snoring, sound asleep on the porch."
"I heard Denmark sound asleep on the porch, snoring"

By correct use of commas you can subtly change the context, but both would be correct.

They are actually both correct. It's just a matter of which one you think sounds better. I prefer the first one.

1st one