My guess is "tiene una voz fuerte/alta"--that is "has a loud voice" For "sounds loud" its would be "suena fuerte". Apparently "ruidoso" can be used for loud, but that appear to not really apply to a person, but rather an event. (I thought it only meant noisy). It looks like voz fuerte is more common than voz alta, since voz alta can mean "high voice". In fact it looks to me like "mas fuerte" is more common than "mas alto", although of course it also means "stronger".