:( Probably not... I believe the MP3 shield has it's own clock (which determines the rate at which the data is read from memory and written to the digital-to-analog converter).
A computer can re-sample the( uncompressed/decompressed) audio stream in real-time to change the speed. But with the Arduino and a shield, the Arduino doesn't have direct access to that stream, it can't change the read-speed, and it's probably not powerful enough to process the audio in real-time, especially if your software is doing something else.
It's possible to alter the MP3 file header to make it play at the "wrong" speed. For example, changing the header in 44.1kHz file to say 48kHz will make it play faster. But that's not a very practical solution, and it's unlikely to give you the exact speed change you want. Note this is the sample rate (kHz), not the bitrate (kbps).