On the site IP Iustitia a lawyer, Jani Ihalainen, discusses this in detail, at least for the USA.
TLDR: in the US the courts have held that a voice is not copyrightable, but copying a voice can still be forbidden in certain circumstances if the copying can be regarded as impersonation. Bette Midler and Tom Waits both won cases where other people copied ...
Your best bet in the classical world may well be Flemish Polyphony.
Missa Virgo Prudentissima, Gloria
A bit of research indicates that the subgenre of late Burgundian motet is the ideal overlap of complex polyphony with a distinct lead voice:
Mass O Crux Lignum Motets Chanson - Antoine Busnois
I've found it difficult to find this elsewhere in classical ...