WHO sings the theme song?

Her name is Nicki Mathis and she will be recording a new version for the sequel Manos Returns and offer it as a digital download reward for the Kickstarter campaign https://www.kickstarter.com/projects/1840778752/manos-returns

Movie Vigilante - Letting movies have it.
http://www.movievigilante.blogspot.com

I assumed it was Hal Warren, since he did just about everything else in this picture.

I adore all the trippy old dusty forgotten obscure music in this film

That song is actually kind of good in how it feels like a “lost” vinyl record one would fine while cleaning out their deceased grandpa’s attic.