Play sound on top of background music using arduino uno tmrpcm library
Assuming you are using an Arduino Uno, You didn't say. As there are many Arduino platforms these days.
As the tmrpcm library is using the PWM  (Pulse Width Modulator) HW (Hard Ware) to recreate the sound waves from the WAV file on the SDCard, the PWM is not available for creating the sounds from your piano application (you didn't say what piano application you were using).  That said, it is not impossible to re-write both applications to SUM the two wave forms together mathematically before resampling the SUM and writing that sample value to the PWM HW to simulate the SUM of the two wave forms.  However, this is likely not trivial and it is also likely the Arduino Uno processor (an Atmel ATmega328P) will not be powerful enough (perform the math fast enough) to keep up with normal sound playback.
It would likely be much simpler to use 2 Arduino Uno boards and sum the sound from the sound file application and the piano application using analog techniques external from the Arduno Uno boards.