Do the high / low levels of an I²C slave's response have to be of identical voltage as the levels of the I²C master's message?
I2C bus levels for the TDA7318 are on page 5 of the datasheet.
It accepts 3V as the minimum logic high voltage.
And besides the I2C bus is open-drain, so it is not the chips that determine the bus high voltage, it is the pull-up resistors that set the voltage, and on the Raspberry Pi the resistors are to 3.3V. There won't be 9V on the bus.