Hi,
You can tell a MediaRecorder element to start recording any time once the trial has started. Just make sure you have stopped your MediaRecorder element by the end of the trial, otherwise you might run into conflicts and/or massive audio files which could easily fail to upload.
Let’s illustrate a situation where you start recording before starting audio playback. Say your MediaRecording starts recording at timestamp 1111, and your Audio element starts playback at timestamp 1234. By analyzing the recorded sample, you find a 450ms silence before your participant starts speaking. This means that your participant gave their audio response 1111+450-1234 = 327ms after the onset of your Audio stimulus. As you can see, the situation is very similar to the previous one, there’s no problem with it.
Best,
Jeremy