Hold that Tape Recorder!
Question: When I go to hear a public lecture I am in the habit of taking along a tape recorder so that I can record the lecture and listen to it again at home. Is there a need for me to request permission from the lecturer?
Answer: There is a basic principle in human affairs that when someone says something to another person he is not necessarily interested in having that information shared by others. This is why our Talmudic Sages ruled (Mesechta Yoma 4b and cited as halacha by Magen Avraham in Shulchan Aruch Orech Chaim 156) that it is forbidden for one to relay to others what he has heard from an individual unless that person grants him permission to do so.
Just as there may be reasons unknown to the listener for the speakers desire for confidentiality in the above-mentioned case of individuals, there is reason to assume that a lecturer addressing a particular audience may also object to having his statements heard by those outside of that audience. There have been many instances where a remark which was "politically correct" for one audience reached outsiders through a recording and was the catalyst for ugly recriminations.
An additional factor to consider is that many professional lecturers have an arrangement for marketing the tapes of their lectures and view the taping by individuals as an infringement on their enterprise.
In conclusion, come early to the lecture and take the pains of asking the lecturer or his escort for permission to do the recording.