Generally passable. The use of possessive pronouns "one's" and "his" do seem off though. One's refers to the speaker or a general audience while "his" is a specifically male pronoun. If the speaker was a woman speaking to either another woman or audience, then this would sound wrong. If this sentence was spoken by a male or addressing a man it would be less noticeable. If you want to retain the sentence structure as much as possible I would use:
"Smile! The beauty of one's soul reflects in their smile."
While "their" is a third person plural pronoun, in common American English it has become very common to use as a singular substitute when the gender us unspecified and would sound more accurate. Keep in mind that some people still debate that this is acceptable; though that is generally only limited to English majors. It has been used in this way for decades in common speech. Considering the hot button debates with trans pronouns these days, you likely won't hear complaints anyways.