The opposite of mellifluous is “strident”, meaning loud or piercing, and harsh in tone. You might use a strident voice when you’re shouting at someone. Depending on your tastes, you might like experimental jazz, but think that punk rock is nothing but strident noise – but the punk rockers may consider your jazz to be equally unpleasant!

Example: “Although the carnival was extremely noisy, we could clearly hear the strident voice of the auctioneer at the animal pens as he sold off the sheep and cattle.”