"最高声部的" can be translated as "of the highest vocal part" or more contextually specifically as "soprano" (when referring to female highest voice in choral music) or "treble" (when referring to the highest male or child's voice, though less commonly in adult choral contexts where boys' voices have changed; more often used in children's choirs).
If you're talking about the highest female voice in a choir, you'd typically say "soprano."
If you're referring more generally to the highest vocal part without specifying gender, "of the highest vocal part" would be accurate, though in musical terminology, specific terms like "soprano," "alto" (for the next lower female voice), "tenor" (for male higher voice), and "bass" (for male lowest voice) are more commonly used.
For a direct translation that fits most contexts where "最高声部的" might be used, "soprano" (if female) or a general description like "the highest vocal line" would work well. If referring to a part in a musical score without specifying voice type, "the highest part" or "the uppermost part" could also be used.