Letters are pronounced in sequence, one following another, while sounds have audible beginnings, duration, and endings. Articulated letters and vocal performances are therefore events in time.
If all letters of a word were present at once with no sequence, they would not constitute ordinary pronunciation. This succession (taʿāqub) makes it problematic to identify spoken sound itself with an eternal attribute wholly free of temporal sequence.
This establishes the temporal character of human pronunciation and vocal expression. The existence of an eternal attribute of speech is a further question to be established by its own evidence.