Deep-mouthed is an adjective that is used to describe a sound or voice that is low, loud and resonant. However, there are several other synonyms that can be used to describe the same quality of sound, such as sonorous, thunderous, booming, or resonant. Other similar alternatives that can be used as substitutes for deep-mouthed are rumbling, throaty, bass, or baritone. These words can be used to describe the sound of a growling animal, the thunderous noise of a waterfall or the low pitch of someone's voice. Using these synonyms can add variety to the language and make the writing more interesting and engaging.