Other suggestion: play WAV files! 
Why bother with MP3? It is good for compression when you want to download songs faster or want to save space on storage. But today's microSD cards can easily store 4GB, that's over 6 HOURS of uncompressed (WAV format) stereo audio at 44.1 kHz sampling rate! You don't really plan on playing police sounds longer than that, right? 
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The Rugged Audio Shield: Line In, Mic In, Headphone Out, microSD socket, potentiometer, play/record WAV files