Translation:The colonel talks with the soldiers.
Technically, you could translate this sentence to what you have posted, but I have found that duolingo typically wants the present indicative form (for now, but I'm sure this will change in later lessons).
That said, in English, there is a distinct difference between the present indicative "speaks" and the present progressive "is speaking". In Spanish, it is perfectly acceptable to use the present indicative (El coronel habla con los soldados) for both meanings, but in English it is not.
"The colonel speaks with the soldiers" implies the colonel speaks with the soldiers from time to time. "The colonel is speaking with the soldiers" means the colonel is with the soldiers at this very moment and is in the process of speaking to them right now. They have very clear and distinct meanings in English and the former may not be used to indicate the latter, unlike in Spanish.
Another small point is that most English speakers tend to prefer "speaks to" over "speaks with". It is definitely not wrong to say he speaks with the soldiers, but it is just more common to hear he speaks to the soldiers.
To summarize the above:
"El coronel habla con los soldados" can translate to either "The colonel speaks with/to the soldiers", or "The colonel is speaking with/to the soldiers". However, "El coronel está hablando con los soldados" only translates to the present progressive form of "The colonel is speaking with/to the soldiers".
For this reason, you will tend to hear native English speakers that are learning Spanish prefer "El coronel está hablando con los soldados" to describe the current action versus "El coronel habla con los soldados" which is more natural to native Spanish speakers.
I agree that "speaks with" and "speaks to" are very close in meaning, and are just about interchangeable, but I think that "speaks with" slightly implies a conversation, but "speaks to" slightly implies a single direction of communication, such as a speech or giving a command or simply stating something.
When you speak with someone, you and the other person take turns speaking to each other.
That is a strange sentence in English, unless it is a change, unexpected or somebody was arguing that the colonel does not speak to them. Otherwise, 'does' is superfluous and sounds strange.
But at any rate, a more clear translation of your suggested sentence would be the equally affirmative “El coronel sí habla con los soldados."
The actual issue in DL is that it does not accept "speaks" in place of "talks" which it logically should due to the synonymic nature of the words and due to the context of a colonel's relationship to his soldiers. Talking and speaking are nearly the same but when an authority figure is engaging with lower ranking people he is likely to speak rather than talk regardless if it is "to" or "with" them.
1) Duo did accept from me "speaks with the soldiers."
2) Duo has accepted, many times from me, the "present continuous/progressive as a translation for the Spanish present indicative. (i.e., is speaking with/to...) This includes several answers in this lesson.
Using the present continuous/ progressive ("is speaking") is entirely appropriate. See these references.
A recent sentence of mine was "the frying pan is ours" i went to the comments section, and many people were making a war reference, where one side had rifles and the other had frying pans. the frying pan side won, and their war cry was "THE FRYING PAN IS OURS!". this is what the colonel is telling the soldiers.
With my limited knowledge of the Spanish language, I'm gonna try to answer your question haha. 'A' means 'to', so the sentence would translate to "The colonel speaks with to the soldiers." Like I said though, I have a limited knowledge of Spanish, so I could be wrong. But I think the answer to your question is no.
I agree, however talk and speak do have slightly different connotations in this context. "Speaks to the soldiers" implies, not strongly, that the colonel is up on the reviewing stand giving a speech. "Talks to the soldiers," implies something closer to the colonel wondering around the night before the battle patting people on the shoulder and saying things like. "You'll do fine, Sam. Have enough ammo Pedro?" What I don't have any idea about is whether that subtle difference in implied meaning is reflected in the Spanish phrase.