Their Circumstances is transcribed as /ðeə sɜːkəmstaːnsɪz/. I do transcription in RP (British English), so the is not voiced unless it is followed by a vowel. Circumstances is also plural, so while the is transcribed as a voiceless /s/, we have to take on a plural ending. As the /s/ is a sibilant, it cannot be followed by another sibilant, so there has to be put a vowel in-between and all vowels are voiced, thus the plural is a voiced /z/.
Scarred Souls is transcribed as /skaːd səʊlz/. Again, /r/ is omitted unless followed by a vowel and here it is followed by a consonant. Souls is plural, and as the /l/ is a lateral approximant it is transcribed with a voiced /z/.
Reunited is transcribed as /riːjuːnaɪtɪd/. This is a past tense ending, and past tense always ends in either voiceless alveolar /t/ or voiced alveolar /d/. As the infinitive here ends with a /t/, a vowel has to be inserted, and vowels are always voiced so the past tense ending is transcribed /ɪd/.
Yesterday’s Tears, the upcoming novel, is transcribed as /jestədeɪz teəz/. Same thing again with the genitive and the plural . Both pronounced with the voiced /z/ because it is preceded by vowels.
Measured Doses, the completed novella I’m getting ready for submission, is transcribed as /meʒəd dəʊsɪz/. Past tense voiced alveolar /d/ in the first word. In the second, the infinitive ends with a sibilant, so here the ending is the voiced /ɪz/.
Inner Demons, the novella I’m currently working on, is transcribed as /ɪnə diːmənz/. The /r/ is omitted because it is not followed by a vowel. The infinitive of the second word ends with the voiced alveolar nasal /n/, so then the plural ending also has to be voiced; thus the voiced alveolar fricative /z/.