There is someone in my shul that mumbles the words. I do not answer amen to his brachos. Is that correct practice?
If his berachos are not iterated correctly, to the degree that they are unintelligible (the words are not said right), you should not answer.
However, if the berachah is said mainly to himself (inaudibly), but the end is audible, you should answer amen upon hearing the end.