i had searched on the dictionnary and i found that both of them are adverbs, but as chinafan said it depend of the sentence; for example when you say he speak aloud it mean ( il parle à haute voie) and when you say he speak loudly it mean (il parle bruyemment)
which mean aloud=à haute voie and loudly=bruyemment hope you understand the french betweem brackets .but i will do another search .ok bye