Reproduction steps:
1. Open the attached project
2. Open the "GetOutputData" Scene
3. Play
4. Notice that GameObject GetOutputData has an Audio Samples array which is full of zeros
5. Press Q to call AudioSource.GetOutputData function
6. Notice that AudioSamples array is still empty
7. Press Q again and notice that AudioSamples array has received data

Actual result: AudioSource.GetOutputData returns an empty array the first time it is called
Expected result: an array is expected to be filled the first time GetOutputData is called

Workaround: Calling this function twice returns audio data as expected

Reproduced on: 5.1.4f1, 5.2.4f1, 5.3.5p1, 5.4.0b19

By design: GetAudioData returns whatever is playing in the history buffer of the sound playing. If it wasn't that, it would have to be a consume operation or part of the mixer thread.

Considering that this is called before the sound is playing, the history buffer is naturally empty.