Hi Upekkha100,
These are just my thoughts;
Sound needs to travel from the person talking to the listener, so a listener closest to the one talking will hear it before someone else sitting further away (even by mili-seconds). So I doubt that the gap between the time that the words are spoken and the time another hears it has any impact.
How about the use of a microphone and a sound system? Would you need to directly hear it from the mouth and would the use of machinery interfere in the necessary conditions? Again, given that most (if not all) sermons these days are relayed through sound systems and the number of people who have attained magga pala by listening to such sermons, we can assume that the use of such sound systems too have no negative impact.
Linking the above two, listening to a recorded sermon should be sufficient. This again is corroborated by accounts of people as well. I am no physicist or scientist so this may not be a good scientific deduction, so please excuse me if this is incorrect.
As another thero (Nivathapa thero) has mentioned listening to Dhamma face to face would be better than listening a sermon live though radio / tv/ youtube. And listening to it live would be better than listening to it recorded.