Simple Audio Sampler And Looper
You should use a signal processor or severals signal processors with MP3 playback capability to do this task. Note that MP3 has a quite a long setup time from start of decoding to hearing sound at the output, and also MP3 decoding is quite an expensive operation in DSP, using quite a lot of memory. This design would be much simpler by using a non-compressed audio format such as PCM.