首楞嚴義疏注經 8.412

8.412

聲能鼓動心海，如波如濤。取此造業，故臨終時先見此也。降注，下流也。

Sounds can agitate the ocean of the mind, like waves and billows. Seizing upon this, one creates karma; therefore at the point of death one first sees this. "Plunging downward" means flowing down.