What's said is that here in Portland (OR) - and probably in some other towns - there's an anti-busking ordinance that prohibits playing a musical instrument in the downtown area if it/you can be heard 50 ft. away.   Meanwhile, a loud-mouthed "preacher" can bellow with enough decibels to be heard 3 times that far, with impunity.

Yes, if there are complaints, the "disturbing the peace" or "creating a public nuisance" laws can be enforced against the vocal ranters, too.   And they probably should be against the loud busker too, if the music's drawing a crowd that blocks the sidewalk or interferes with local businesses.   But I don't think the content of the "speech" or the type of noisemaker (vocal, instrumental, poetic, prosaic) should be the criterion.