Adam Clarke on John 12:29

The people - said that it thundered: others - an angel spake to him - Bishop Pearce says, Probably there was thunder as well as a voice, as in Exo 19:16, Exo 19:17, and some persons, who were at a small distance, might hear the thunder without hearing the voice; while others heard the voice too; and these last said, "An angel hath spoken to him."

Wetstein supposes that the voice was in the language then in use among the Jews; which the Greeks, not understanding, took for thunder; the others, the Jews, who did understand it, said it was the voice of an angel. In Rev 6:1, the voice of one of the living creatures is compared to thunder; and in Rev 10:3, the voice of an angel is compared to seven thunders. The voice mentioned was probably very loud, which some heard distinctly, others indistinctly; hence the variety of opinion.