This is a rather complex topic, so rather than giving a list of situations where the personal (inflected) infinitive is used, it's important that you first understand the main difference between the inflected and uninflected forms: only the inflected form can have an explicit subject (minus one exception, not relevant here). When the uninflected form is used, its implicit subject depends very much on the situation. It can have a generic interpretation (any person, as in comer rápido faz mal) or it can reference another constituent of the sentence (which one, depends on the type of construction; for instance, with auxiliary or semiauxiliary verbs, as in vou comer sopa, the subject is the same as the subject of the auxiliary verb).
With just these considerations, you can see that this sentence is wrong:
*Tens que falar mais alto para os vizinhos te ouvir.
The infinitive clause has an explicit subject (os vizinhos), therefore the inflected infinitive is mandatory. If you write instead:
Tens de falar mais alto para te ouvir.
The subject of ouvir depends on the context (it could be 1st person singular or 3rd person singular). This is on the surface ambiguous between the uninflected and inflected infinitive, since the two have the same forms in 1ps and 3ps, but it's actually an inflected infinitive, which these subordinate clauses with para require. If you were talking on behalf of a group, you'd have to say para te ouvirmos. You can disambiguate the sentence above by including an explicit subject, such as eu / ela / o vizinho. Note that, even if we make the subject of ouvir not explicit, its value doesn't depend on the subject of the clause the subordinate clause is inserted in.
Unfortunately, these considerations alone are insufficient to address your other sentences. Let's start with this one:
*Estamos a falarmos aos clientes.
In this case, we have a verbal periphrasis with the auxiliary verb estar. The inflected infinitive cannot be used with auxiliary or semiauxiliary verbs. The infinitive does not form its separate clause here. Another situation where the infinitive is mandatory is in the fazer-inf constructions such as ela fez-lhes vender a casa, in which (arguably, though) there is only one clause.
Because ter que is a modal semiauxiliary, the sequence tens que falar in tens que falar mais alto also cannot be replaced with *tens que falares.
Finally, in this pair, both are correct:
Ajude as crianças a transformarem-se em adultos.
Ajude as crianças a transformar-se em adultos.
The verb ajudar requires a mandatory control construction from the direct object of the main clause, which means the direct object of ajudar (here, as crianças) has to be the subject of the infinitive verb. Other verbs in this class include aconselhar, animar, convencer, convidar, forçar, proibir, and so on. Despite this restriction, the inflected infinitive can still used. Oddly enough, an explicit pre-verbal subject is prohibited (though it could be after the verb):
*Ajude as crianças a elas transformarem-se em adultos.