No one can know if you need speech therapy without first having heard your voice. And even if we on HN could hear your voice, most of us aren't speech therapists so how should we know?

That said, there is definitely an autism voice. It sounds "throutish" and is especially apparent among women. Perhaps because non-autistic women in general tend to have more melodic voices. However, that is a distinctive way of speaking and not a speech impediment in itself.

So unless you believe that employers are discriminating against autistic candidates, altering your voice won't help. But perhaps you can try and speak louder -- it is often seen as a marker of confidence.