The MP3 ans SD use SPI the RTC uses I2C.
That does NOT mean that the MP3 shield doesn't also (mis)use other pins. If it uses the I2C pins for ANY reason, that will cause issues when you need to use them for I2C.
The MP3 ans SD use SPI the RTC uses I2C.
That does NOT mean that the MP3 shield doesn't also (mis)use other pins. If it uses the I2C pins for ANY reason, that will cause issues when you need to use them for I2C.