Right
益 supplies the sound - yì unchanged.
益 itself is etymologically the same word: it pictures
水 (water) over
皿 (vessel), originally meaning 'water overflowing a bowl'.
溢 is the modern character that took over that literal meaning, while
益 specialised to the figurative 'benefit, increase'. So
溢 = water + (water-over-vessel) - a doubled overflow image.