Hello!
I found a similar issue under MasterController Issues but it was never answered.

I have a third-generation teen in my active family; she has two valid moms who are in my family and still alive. I have tried to change her voice to be more like her werewolf mother's (rather low-pitched), however it keeps getting set to 1/2/3 at random and the exact middle of the slider. I haven't nailed down if it happens at a particular time of day, but it definitely doesn't persist for long. I am using MC > Advanced > Edit in CAS to "fix" it, but it doesn't stay fixed.

Has anyone dealt with this before or know what might be causing it?

Thank you!

It might be that SP is set to "inherit voice" and so it's changing it to be like mom's?

I will check that when I get home. I think I do have that setting on, but I wouldn't expect it to affect the active family -- or to happen at any time other than birth.

The only reason I say this is because I did have this issue before where my sim's voice kept changing, and it was because I wanted it different from her mother, but I did have SP's "inherit voice" setting on. It was so weird, some days it would change, and some days it wouldn't.

It would be this option if she's teen or below. Once they are young adult it stops checking.

I had to turn off the "inherit voice" for that reason. It would always get me irritated that not only would it change the voiceset randomly as well as every time they changed clothes (especially at the chemistry set) but it would give them the voiceset that grated on my nerves the most. I have no idea WHY I would always get that voiceset because neither parents had it and the parents were the first gen so no grandparents. Unless it was trying to create a fictional grandparent to pull the voiceset from...?

@Chain_Reaction: Yep, that would explain it! I have added a note to the SP wiki stating that this occurs daily and includes the active family.