https://www.quora.com/Why-does-an-African-American-persons-voice-have-a-different-timbre-from-a-Caucasian-voice
Why does an African American person's voice have a different timbre from a Caucasian voice?
What is physically different to cause a different quality to the speaking/singing voice?
Size/thickness of vocal chords/folds-Since the air must pass through the folds in order to make a sound,the shape and thickness of the folds determines quality of voice as well as range and vocal type. A soprano and an alto singing the same note will have different timbres-the soprano A above middle C for example, will not be as heavy sounding as the mezzo singing that same A, and a tenor singing that same note will have a different sound as well. Lower voices tend to have thicker folds than higher voices, and men have thicker folds than women. Thinner folds means that the chords are able to vibrate faster-thus producing higher pitches. Thicker folds cannot vibrate as fast, which means an ability to produce lower pitches easier than higher voices.
Size of the neck/head (the laryngeal/pharyngeal tract)- The size of the area where the air travels from the lungs to the mouth area impacts the sound of the voice. The head and neck are resonators-places where the sound vibrates and projects. The size of the resonators impacts the way the voice sounds.
Shape of the head/neck/jaw-Shape of the head and mouth and neck are also factors in deterimining how the voice will sound. Different shapes of these areas means the voice will resonate in a different way with different people.
Cultural speech factors-Differences in speech, pronunciation of words, accents, etc, also play a role in how lyrics are sung.
So, it's not so much race as it is physical makeup of people. However, generally, African-American people appear to be physically bigger than Caucasians, and have different head and neck shapes, so it would follow that any differences between the two races would be more because of this rather than due to the fact that one is black and one white. If you listen to two opera singers who are the same voice type or fach, one black and one white, you cannot really detect any difference between their voice timbre that can be solely ascribe to race.