Quoted from the book (Man after Death) By Mohammed-Ali Hassan Al-Hilli

The speech is what man says, so that the second person will understand, whether it be with a loud or a low voice, or with mere signals; as one of the Arab poets said:
" Our eyebrows expressed our mutual feelings:
we were silent, but the love was speaking!"
God - be exalted – said in the Quran 36: 65
اليَوْمَ نَخْتِمُ على أفْواهِهِم وتُكَلِّمُنا أيْديهِمْ وتَشْهَدُ أرْجُلُهُمْ بِما كانُوا يَكْسِبُون
The explanation:
(Today, We set a seal on their [spiritual] mouths; and their hands speak to Us, and their feet bear witness as to what [sins and crimes] they were earning.)

The aim of the 'speech' is to let the second person understand what the speaker wants; while the 'utterance' is the speech that has a sound so that the second person will understand. And if it had no sound, then it is not called an utterance. However, we can say that the man speaks, as can we say that the man utters; but we cannot say the souls utter; because their speech hasn't any audible sound, but we can say that the soul speaks.

God - be exalted - said in the Quran 53: 3-5
وما يَنْطِقُ عَن الْهَوى . إنْ هُوَ إلاّ وَحْيٌ يُوحَى . عَلّّمَهُ شَدِيدُ القُوَى
The explanation:
(Nor utters[5]he[: Mohammed] out of caprice.
It is not but a revelation [from God], that is being revealed [to him by Gabriel, the angel.]
[Mohammed] is taught by [Gabriel: an angel] mighty in power.)

The meaning:
>> (Nor speaks he[: Mohammed] out of caprice) means:
Whatever Prophet Mohammed - peace be on him – speaks with clear speech, and let you hear eloquent utterance [and good style], then all that is not from his own, but is from God - be exalted – which He has sent to the Prophet with Gabriel who taught [Mohammed] the Quran; as it is evident from His saying – be exalted –
إنْ هُوَ إلاّ وَحْيٌ يُوحَى . عَلّّمَهُ شَدِيدُ القُوَى
i.e. (It is not but a revelation [from God], that is being revealed [to him by Gabriel, the angel.]

[Mohammed] is taught by [Gabriel: an angel] mighty in power.)

God - be exalted – said also in the Quran 77: 35-36
هذا يَوْمُ لا يَنْطِقُون . ولا يُؤْذَنُ لَهُمْ فَيَعْتَذِرُون
The explanation:
(This is the day when they cannot utter [any audible word,]
Nor are they permitted [to talk] so they may apologize.)

They haven't the ability of utterance and articulation because they are ethereal souls; moreover, they are not allowed to do so in order that they may then apologize.

God - be exalted - said in the Quran 27: 85
ووَقَعَ القَوْلُ عَلَيْهِمْ بِما ظَلَمُوا فَهُمْ لا يَنْطِقُون
The explanation:
(And the word [of punishment] will be sentenced against them, because of their wrong-doing; and they will not utter [any audible word.] )