It is tricky to use a display that uses SPI for communication, the SPI pins are not available on the unpopulated header that’s available on Pisound itself, see the pinout. If possible, you should use I²C one, the pins are available in the header pinout, or an HDMI one like here.
If you do want to go with SPI one, then you have to solder directly on top of Pisound RPI header. Use CE1 pin for the display’s SS pin, the CE0 is already used by Pisound.