You are printing the values two different ways. In one case, you let Serial.print() decide how to print the value in SdPlay.fileinfo.ActSector. Since ActSector is a uint32_t, or unsigned long, Serial.print() prints the value as an unsigned long.
In the other case, you are using sprintf() to convert the value to a string. But, you are using the %u format specifier, which is NOT the correct specifier for longs. %ul is.