I think there may be a crowing difference between true bantams (that don't come from a standard breed) and bantams that are mini versions of a standard breed (e.g. cochins, brahmas, RIRs etc.).
The standard-derived bantam roos I had weren't like my little "true bantam" breeds -- all of which had variations of the "er-er-ER-er" 4-note crow (I have Nankins, OEGBs, Chabos and Belgian quail D'anvers, all true bantams).
When I had cochin bantams though, the roos sounded just like their standard-size brethren except a bit higher pitched and not as loud. My cochin bantam roos had a 3-note, drawn out, deeper crow that sounded like the first notes of Beethoven's Fifth Symphony... with one of the middle notes missing: "Ba-Ba-BAAAAAAA."