Potential synonyms for the phrase "opulent voice" might include rich voice, luxurious voice, melodious tone, or even sumptuous vocalization. These terms all connote a sense of richness and fullness, suggesting someone who speaks or sings with great depth and expression. Other possible synonyms might include sonorous voice, lustrous timbre, or dulcet intonation, each of which conveys a slightly different shade of meaning but all of which capture the sense of someone with a powerful and captivating voice. Whether used to describe a singer, speaker, or actor, these descriptions can help bring more vividness and specificity to a piece of writing.