The gain parameter appears in audioBlockFormat.

The gain parameter is a linear gain and controls the level of the audio signal in the object. At rendering the level of signal will be multiplied by the gain value. If the gain parameter is not set a value of 1.0 is assumed, so the audio signal’s level is not adjusted.

Ideally, the waveform that is being described should be at the desired level, so the gain parameter is not required (or set to 1.0), rather than relying on the gain parameter to adjust levels.

When using the gain parameter in audioBlockFormat it can be used for fixed adjustments of the audio signal, and is useful when a single audio track is being used by more than one channel definition (for example, a single sound effect may be used by two different audioChannelFormats, but each requires different levels).