Playing a small MP3 file
I noted that the mp3 file size is 52kB and I heard very little reduction in quality when I down-converted it to a 8 bit, 16k samples per sec mono wav file - it became 48kB long and this is much easier to playback.

How much memory can you afford to have on-board? Surely this is easier to handle than an MP3 file. Wondering why the wav file is smaller than the MP3? MP3 (as far as I am aware) encode the biggest amplitude frequency in a set of dozens of compartments across the audio range. As your signal is largely in the 2kHz to 4kHz area there is a lot of overhead mp3 transmission that is pointless and a useless overhead. Wav is smaller because I've tailored it to the actual audio.

I also note that your audio file is largely symettrical about its half-way time point. I reversed the whole file in wavelab and it sounded pretty-much exactly the same

This will further allow you to halve the audio size - you are now at 24kB.

You might also consider that when the button is held down long enough an appropriate middle section is repeated.