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?
No - even in Shul you only answer Amein if it is done immediately or else it is an Amein Yesoma. Therefore you should not answer Amein if you are listening to a pre recorded Shiur. However if you are listening to a live Shiur then I do not know whether you should answer or not.
No, but if it is live yes (Yalkut Yosef, Helek 3, 215:4). However, over the internet it would be Asur (Q&A 91813 to R' Musafi). The reason is because the internet is a little off.
Forgot to mention: Even if you both are Mechawen you can't be Yose.
The Rivevos Ephraim 8:102:2 writes that watching the syium hashas (live)on tv and hears kaddish ,one can answer amen yehei shmei rabbah... Since it is not the shem Hashem,he also talks about hearing thunder over the phone and holds one may say bracha without Shem UMalchus.
Although if the shiur was live, it could be allowed (you don't need to hear the bracha to answer amein, like in the famous shul in Alexandria where they used to wave flags to tell people to answer Amein)