According to this answer citing RaM"A (124:11), you can answer Amen to any Berachah even if you don't hear it, so long as you know which one is being said. Does this apply only to Tefillah BeTzibur ...
May one be yotzai listening to havdala over the telephone?
Many times I listen to recorded Shiurim (lectures) where the speaker says a Brocho (usually Shehakol on a drink). Should one answer Amen to such Brochos?