Jon's voice doesn't seem to be much weaker, but he has changed his style of singing to fit with the album theme. For example, SSW/NJ will have the louder Jon, TD/Crush will have the softer rocking voice.

I don't think I've ever noticed Bono's voice change in style or in strength. Steve Tyler's has changed in style, listen to the earlier stuff (Dream On) his voice sounds very different to a song like 'Pink'. Thant's what I say anyway!