citing Gabriel Bouvigne bouvigne@mp3-tech.org:

Could someone explain why the multiply and accumulate instruction seems so important in Mad?

I can't speak for the MAD project specifically, but the "mac" instruction is commonly available and used on DSP processors. Often they can do the instruction in one CPU cycle, which can be used e.g. for FIR filter or a FFT implementation. Maybe the IMDCT in MAD uses a similar approach.

bye Michael