What is the electric potential of the heat sink (metallic surface) on back side of IC package? Is that a GND?

The heatsink (metallic surface) on the package backside is connected to substrate (lowest electric potential) on the audio power IC chip backside through a conductive mounting material as shown in Fig. 1. Unintended operation may be appeared if the electric potential of the substrate becomes high. The IC can operate correctly even if this heat sink is open, but the heat sink should be connected to the lowest electric potential (GND) of the audio power IC if it is connected.

However, if the lowest electric potential (GND) is unstable due to the unsuitable pattern layout and so on, the performance and sound quality may be affected.