Calculate a 'fade' value (i.e.\ 'balance' between front and rear)
* for the specified volume with the specified channel map. The return
* value will range from -1.0f (rear) to +1.0f (left). If no fade
* value is applicable to this channel map the return value will
* always be 0.0f. See pa_channel_map_can_fade(). \since 0.9.15
Calculate a 'fade' value (i.e.\ 'balance' between front and rear) * for the specified volume with the specified channel map. The return * value will range from -1.0f (rear) to +1.0f (left). If no fade * value is applicable to this channel map the return value will * always be 0.0f. See pa_channel_map_can_fade(). \since 0.9.15