In fact, we had troubles from day to day with people's voices. One member became sick, and the frequency spectrum of his A's changed. No, the changes were too great. This brought to light certain other charactertistics of the human voice. Singers have different voices, like chest voice and head voice, and each voice may have different frequency characteristics.
Voices also change with stress.
The audible signs that someone is under stress affect the speaker's frequency spectrum. These changes would tend to affect the identification system. In general, accounting for the changes just within our group's voices was our number 1 concern. The task of identifying speakers based on the frequency components of their voices was a complete success.