(I hail from England)
So I tried it on:
"I wrote a letter regarding Zebra's taking herbs, placed it in an envelope and posted it on schedule."
The first time it gave me an overall score of 20, and imagined that I was Australian. The third time an overall score of 10, and imagined that I was from the US. Both times I used the same form of pronunciation.
First:
Overall: 20
Words: "I"1 "wrote"34 "a"1 "letter"34 "regarding"25 "Zebra's"7 "taking"40 "herbs"0 "placed"28 "it"50 "in"50 "an"42 "envelope"15 "and"34 "posted"7 "it"1 "on"0 "schedule"0
Accent: US1% UK1% AU91%
Exam: IELTS: 3 TOEFL: 0-11 CEFR: A1A2B1B2C1C2 PTE General: 1
Third:
Overall: 10
Words: "I"0 "wrote"16 "a"1 "letter"1 "regarding"34 "Zebra's"8 "taking"17 "herbs"1 "placed"9 "it"42 "in"22 "an"1 "envelope"0 "and"0 "posted"7 "it"21 "on"1 "schedule"0
Accent: US76% UK5% AU3%
Exam: IELTS: 2 TOEFL: 0-11 CEFR: A1A2B1B2C1C2 PTE General: 1
I used "Zeb-ra" (not "Zee-bra"), "Her-bs" (nor "Er-bs"), "On-ve-lope", and "Shed-ule". I was speaking in my natural manner, without mixing throwing in any weirdness.
(In my second attempt, I used "En-ve-lope", and got an even lower score.)
Overall, I have to same I am not too impressed.