I must agree with the comments that the "four types of conditional sentences" are badly misleading and do not accurately reflect how conditionals are in fact used, or how grammarians discuss them. That said...
What is the meaning of the last sentence (or in what situations we can use this sentence)?
If I spoke louder, you would have heard me before.
The phrase "If I spoke louder" (using a simple past tense) could refer to the present or to a hypothetical future. (this phrase is the premise of the conditional) But when combined with "would have heard" it seems to be referring to an unreal or hypothetical past. The whole sentence I take to mean that the narrator said something, but was not heard the first time that s/he said it. Now the narrator speculates that if that past speech had been louder, the person addressed would have heard it. So this form can be used when speaking of a possible past event that did not occur (an unreal past) and stating a conclusion about what result would have followed had the event in fact occurred. Implicit is that the subsequent event did not in fact occur.
"Before" could be replaced with "before this" or "earlier" and the result would have the same meaning, but in my view would be clearer.
What is the difference between the above sentence and the sentence:
If I had spoken louder, you would have heard me before
There is no significant difference in meaning. This form, by using "had spoken" (past perfect) makes it clear that the sentence deals with an unreal past. One need not analyze the clause describing the consequence or conclusion to realize that this is about an unreal past, use of the Past Perfect form hollowing an "If" tells us that. Thus I would often prefer this to the form using the simple past. It may sound slightly more formal than the form with the simple past.
This is an example of what some call a "Past counterfactual state, past hypothetical consequence" See "Conditional Sentences in English: Counterfactual Conditionals
where the example (#52) given is "If it had been fine yesterday, we would have had a barbecue" Others refer to this as a subjunctive sentence, although it dose not use the grammatical subjunctive mood. It is also known as a conditional perfect
Note that in both sentence the hypothetical conclusion is implied to be certain or highly likely if the premise had been true. If the conclusion had been less cerftian, a form such as:
If I had spoken louder, you might have heard me before
could be used.
