Can you answer amen to a Baracha that was made over the telephone?
See which concludes that Rabbi Shlomo Zalman Aurbach Zatzal held it was not permissible.
Yes. (Yalkut Yosef Helek 3, 215:4).
yes and also ieh shemeh rabah, and I think 13 midot and kedusha also, not because you heard it (because you didn't) but because you know someone finished a beracha.
this would also apply in a place that people say amen loud before the hazan finishes the beracha, technically you didn't heard the ending but you can say amen because you know he did.
note that live radio would be the same