Always use microphones and hearing loops
If you have microphones, connect them to the hearing loop and make sure that everybody uses them.
A lot of venues will have audio induction loops for the benefit of the hard-of-hearing (it’s required by law in the UK). Provide microphones for anybody who’s speaking, and connect them to the hearing loop.
Remind your speakers to use the microphone, even if they think they can project their voice – it makes it much easier for people who are hard-of-hearing. If you have a Q&A, repeat questions into the microphone, or even better, have a roving microphone. If you’re recording the sessions, using a microphone means you get much better audio quality.
Remind people not to tap or blow into the microphone – the sudden noise can be painful to anybody using a hearing aid, as the sound goes straight to their ears.
Asking “can everybody hear me” into a microphone is pointless, because anybody who can’t hear you won’t hear the question! If you have to ask, it’s better to put the question on a slide in big text.
For much more about audio setup – things like mic options, battery packs, and common gotchas – I recommend reading Nat Torkington’s guide to inclusive event audio.