The first thing I'd look for upon hearing that about someone is major placements in Taurus. Taurus rules the throat, including the vocal cords. You've got one: Taurus Moon.
Next thing I'd look for is a connection between Venus and Mercury. Yours are sextile.
Since you always hear it from other people, those other people are always (or nearly always?) female, and you don't believe it about yourself, I really give credit to that Taurus Moon. Moon = women; at the DC = comes from others. It's something others give you that you can't give yourself (you don't like the sound of your own voice, but they do).
For what it's worth, some women (who are attracted to men) are more attracted to men based on their voice than on their looks. I know because I'm one of them. Looks only play a small part in attraction for me. Voice has a much bigger role. If any of these women who compliment you on your voice are also that way, that's saying something.