'sweet-sounding' is a Modern English compound formed from 'sweet' + the present participle 'sounding' (from the verb 'sound').

'sweet' originates from Old English 'swete' (from Proto-Germanic *swōtaz) meaning 'pleasing'; 'sound' (verb) traces back to Old English forms meaning 'to make a noise' and both elements were combined in Modern English as a compound adjective.

Initially the elements meant 'pleasant' and 'to make a sound'; combined they literally meant 'making a pleasant sound' and this core sense has largely remained, now expressed as 'pleasant to hear'.