What term describes a sound that is loud and resounding?

The term that best describes a sound that is loud and resounding is "resonant." When a sound is resonant, it means that it has a rich, deep, and clear quality, often causing it to sound fuller and more powerful. This can apply to both musical notes and general sounds that carry well or fill an environment boldly.

While "plangent" does describe a sound, it carries a connotation of being mournful or sorrowful, which does not align with the idea of just being loud and resounding. The word "melodic" relates specifically to melody and tunefulness rather than volume or resonance. "Echoic" refers to the quality of producing echoes, which is more about the repetition and reflection of sound rather than its inherent loudness or resonance. Thus, "resonant" is the most fitting choice for a sound that is both loud and resounding.