How to answer tough questions / didn't hear the question?
Maybe it depends on the discipline, but I have usually found the conference audiences supportive, especially toward young researchers. In the case of tough question I would explain that the question is indeed tough and answer those aspects I understand and point out that the problems posed by the asker should pose an interesting venue of future research. Just learn from this experience, as conference attendees are generally smart people and probability of faking out a plausibly sounding answer is low when one just doesn't know.

As for the second problem I don't think gambling is a good solution. It'd better to explain hearing problems and ask to repeat the question. After all you want to address the issues pointed out by audience and maybe learn something from it. In grave situation maybe you can ask a colleague to write the question summary down for you? Informing session chair of your hearing problems beforehand won't hurt as well. After all that's nothing one should be ashamed of (as already pointed out by others).