Green’s Dictionary of Slang

bass n.³

SE in slang uses

In phrases

get the bass out of one’s voice (v.) [one trad. lowers one’s voice when one is being verbally threatening]

(US prison) to stop acting aggressively.

1992

Bentley & Corbett Prison Sl. 93: Get the Bass Out of Your Voice An expression to tell someone to quit acting aggressive or forceful.