'sound-amplifying' is a compound of 'sound' and 'amplifying'. 'sound' originates from Old English 'sund' meaning 'noise' or 'sound', and 'amplifying' comes from Latin 'amplificare' via Old French and Middle English; 'amplificare' is formed from 'ampli-' meaning 'large' and 'facere' meaning 'to make'.

'amplificare' (Latin) became Old French/Medieval forms (e.g. 'amplifier') and entered English as 'amplify'; combined with the modern English word 'sound' this produced the descriptive compound 'sound-amplifying' in modern English.

Initially 'amplify' meant 'to make larger or greater' in a general sense; over time it specialized in many contexts to mean 'to increase intensity or volume (especially of sound),' which is the sense used in 'sound-amplifying'.