In music, voice refers to several melodic lines being sung or played at the same time. These lines are called "voices", even if they are not being sung but being played on instruments. Another word for the same thing is "part", but the word "part" can also mean a "section" of a piece, so it can be confusing.
The word "part" meaning "voice" is often used when describing polyphonic music. For example: a 3-part fugue would have three "voices". It might be sung by three people, or it might be played by three instruments, or just by one keyboard instrument playing all three parts.