to smelt, to melt
烊 centers on applying intense heat to transform a solid into liquid: smelting ore or melting a substance.
烊 combines 火 (fire) with 羊, which likely contributes the sound. The character expresses melting or smelting by fire.
Fire 火 on the left and a sheep 羊 on the right: imagine using intense fire to melt sheep fat or smelt metal, turning solids into liquid.
For ヨウ, picture a sheep-shaped candle melting under fire: the sheep says 'yo!' as it melts, cueing ヨウ.