Note that the word “aloud” is not in the Greek; the ESV translators have added it probably to indicate that the lament of Revelation 18:16 was audible (as opposed to a whisper). John had already indicated in Revelation 18:11 that the merchants were weeping and mourning. The repetition serves to stress the point. It also gives colour to the lament of Revelation 18:16; it is an emotional lament voiced through tears.
15 The merchants of these wares, who gained wealth from her, will stand far off, in fear of her torment, weeping and mourning aloud,