Duval' voice changes way too much in those first five episodes. He goes from gravely when introduced to no gravel to accent to… There's just no consistency. Hopefully he sticks with the pretty it voice for the second half of this set.
I’m not sure I fully agree with your assessment. His voice did change quite a bit, but I’m not sure I would say there was no consistency; and by that I mean the voice wasn’t consistent, but I think why it kept changing was consistent, in regards to the character.
When he first shows up and, in general, the first couple scenes he is in, he does have a more gravelly voice. He is meant to come off as more intimidating and scary because he is under the persona of “Iron-Masked Duval”. Because of the government hunting him down, he was basically forced to take on another persona entirely; he was forced to go from small-time thug to the threatening leader of a known kidnapper group. He really starts to lose the deeper tone in his voice once he comes face-to-face with the Strawhats, though he is still a bit raspy. At this point he even directly says that if they kill the Strawhats they will no longer need their base for their operation anymore; he is basically starting to shed the “Iron-masked Duval” persona. And once the helmet is removed we start to hear more of what the real Duval sounds like, who has a more normal sounding voice (only a bit raspy at points).
And as for the accent, like Maron pointed out, this was something that was not always consistent. It is established that he starts to get an accent when he is particularly upset or angry; it’s his quirk. It first comes out not too long after he heard about the Strawhats being near his base. After that it comes out when he comes face to face with Sanji (the man he’s most frustrated at), when his mask is removed and goes on to explain how terribly upsetting it is to be confused for Sanji’s poster, and when he goes into his final angered monologue right before Sanji kicks his face in.
So basically he is transitioning between two separate personas, with the accent being a quirk of both personas. Also I do seem to recall Duval having a more consistent voice once he becomes handsome.