For those that entreat other beings, such as clerics, paladins, druids, and arguably warlocks as well, the need for vocalisation makes sense, absolutely, but the need for it to be forceful, precise and loud very distinctly does NOT. .
I personally think Warlock are basically mages. They learn magic from a teacher and both use the rules of arcane magic to cast their spells. The main reason pact mages get a bad name is because wizards are jealous. Why are you a half-baked magic user just because your teacher actually understands magic (Warlock) instead of just being a slightly more experienced diletant (wizard)?
Puting that aside, I remembered that in Xanathar's there are some nice customization option for classes. One of these, for the Pact Mage, has that s/he shouts the name of their patron together with the spell. If you go from there, it could be better to have your mage comment on his spell action, after a short cantation in medium volume.
Flama Flamae Flamae Flamam Flama [normal voice]
Burn in my arcane might! [shouted]
The problem is, that many spells would need a custom shout and it needs to be fitted to different classes.
While I would find such a thing more flavourful, I assume the shouted words are just easier and cheaper to produce.