Actually, fanmade variants like Neru Akita and Haku Yowane do not even have voices and settings for their voices heard in videos. It is up to producer preference for either character and others like them. Crypton has shown no interest in making voices for the characters either as they are just variations of their own characters, representing fan thought process and workings.
Another answer: Yes, Haku and Neru do sing. They don't have official voicebanks, but some people try to make them sing by using Miku Append. For the songs that aren't append, people usually make Haku a lower Miku and Neru a higher Miku or lower Rin, although some people believe that Neru is a tomboy so she should be a lower Miku.