Princeton's WordNet

1. loud(adj)

   characterized by or producing sound of great volume or intensity

   Synonyms:
   flashy, tawdry, cheap, meretricious, tacky, flash, gimcrack, tatty, garish, brassy, trashy, gaudy, forte

   Antonyms:
   subdued, soughing, susurrant, hushed, muffled, muted, tasteful, small, softened, euphonious, murmurous, little, murmuring, rustling, soft-spoken, quiet, low, dull, whispering, gentle, low-toned, soft-footed, susurrous, soft

2. brassy, cheap, flash, flashy, garish, gaudy, gimcrack, loud, meretricious, tacky, tatty, tawdry, trashy(adj)

   tastelessly showy

   Synonyms:
   forte, crummy, brazen, brazen-faced, chintzy, ratty, audacious, specious, loud, punk, shabby, flashy, meretricious, jazzy, cheapjack, bodacious, inexpensive, tinny, bum, cheap, sleazy, trashy, brassy, insolent, gaudy, garish, tatty, shoddy, sporty, cheesy, tawdry, brasslike, moth-eaten, bald-faced, barefaced, gimcrack, chinchy, flash, showy, tacky, gilded, rubbishy

   Antonyms:
   subdued, dull, susurrous, euphonious, muffled, soft-footed, murmuring, whispering, muted, little, susurrant, hushed, tasteful, murmurous, quiet, low-toned, soft-spoken, soft, rustling, small, gentle, low, soughing, softened

3. forte, loud(adverb)

   used chiefly as a direction or description in music

   Synonyms:
   flashy, tawdry, cheap, garish, tacky, flash, gimcrack, tatty, loud, meretricious, brassy, trashy, gaudy, forte

   Antonyms:
   tasteful, dull, euphonious, gentle, soughing, softened, soft, murmurous, small, susurrous, murmuring, whispering, susurrant, soft-spoken, muted, quiet, subdued, hushed, little, low-toned, rustling, muffled, soft-footed, low

4. loudly, loud, aloud(adverb)

   with relatively high volume

   Synonyms:
   clamorously, loudly, forte, obstreperously, loud, aloud, out loud

   Antonyms:
   soughing, susurrant, euphonious, susurrous, whispering, low-toned, murmuring, small, soft-footed, muffled, hushed, little, rustling, tasteful, subdued, soft, gentle, quiet, low, murmurous, dull, muted, soft-spoken, softened