My idea for why pony Miku has a somewhat artificial sounding voice is because she was born without a larynx (fancy word for voice box) but desperately wanted to be a singer, so she had a special device made to give her a voice, albeit an obviously artificial sounding one. despite or perhaps even because of her unique voice she became a renowned pop star in Equestria.
Not exactly the most compelling of head cannon stuff but I thought it was a neat little idea to explain why pony Miku's voice sounds the way it does.
Version without the head cannon stuff and a transparent background. >>2631266