The array given in the samples parameter will be filled with the requested data.
AudioSource.GetOutputData
provides access to audio data from a short history window (for example, the last few milliseconds) for analysis purposes. Unity doesn't automatically allocate the buffers required to store this history because doing so would be expensive and memory-intensive. Instead, Unity only allocates buffers and starts to record when you first call this function, on a per-object basis. As a result, the output data will initially be empty until the engine processes sufficient audio to populate the buffer. Please note this function isn't suited for critical or chronological, real-time data analysis or processing, or scenarios where you require low latency.