The Collaborative International Dictionary of English v.0.48:

Loud-mouthed \Loud"-mouthed`\, a.
   Having a loud voice; talking or sounding noisily; noisily
   impudent or offensive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

loud-mouthed
    adj 1: given to loud offensive talk

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "loudmouthed":
   bawling, blatant, boanergean, boisterous, brawling, brazen-mouthed,
   clamorous, clamoursome, crying, full-throated, loud-voiced,
   multivocal, noisy, obstreperous, openmouthed, screaming, shouting,
   stentorian, strident, trumpet-tongued, trumpet-voiced, vociferant,
   vociferating, vociferous, yammering, yapping, yelling, yelping