You obviously want to avoid a situation where your audience can see you but not everybody can hear you. Some say that every person in the room acts as an open window.
Every person in the room will "absorb" some of the sound. This means that the more people in the room the louder you need to speak to ensure your entire audience can hear what you are saying. There is obviously a limit in regards to how loud you can speak. At one point you should consider using a microphone.
There are multiple tips and tricks in this regard to ensuring that your audience can hear you. Take advantage of the tips to ensure that your presentation / speech is being heard (you are in control) and tips to check that your audience is able to hear you. Read all about it in the book that forms part of the Public Speaking Kit.