If there's a microphone on hand, then use it-or at least appear to use it. People think they can hear better if they see a speaker standing in front of a microphone.
Wherever you are going to speak,enquire beforehand if a microphone is to be provided and if it is then test it in advance. It won't enhance the start of your speech if you have to open with the words ‘Testing... Testing... One,two,three, four..