On the Safari browser on OS High Sierra you may have trouble getting your microphone to capture sound when doing HTML5 audio recordings. In this situation the audio will appear to record. Yet when you play back the recording, there is only silence for the entire length of the recording. This seems to be a bug with High Sierra or Safari. We  confirmed this using High Sierra 10.13.3 and Safari version 11.03. But it occurs on earlier versions too.

In our tests were sometimes able to restore audio capture to Safari. We discovered that Safari only seems to use the first audio device on the system. This is not necessarily the selected device selected in the system settings. And it is not necessarily the device selected in the Poodll recorder either. But by plugging in headphones with a mic device to the Mac earphone jack and playing with the volume control for the input devices in the system sound settings, testing recording,  unplugging the device and retesting, and repeating, we were able to jumpstart the audio capture. Oddly it would often work when the volume indicator on the system sound settings failed to work. 

While this is really not a solution for end users it may be of some help to support staff and admins.