Question

why we use the DCT and IDCT functions in matlab for audio compression ?what DCT and IDCT do?

Answer 1

DCT (Direct Cosine Transform) is a special case of the Fourrier Transform. IDCT is the inverse.

(At least some) audio compression algorithms work by removing e.g. a bit of information in the high frequencies (since we're not as attuned to those frequencies as we are to others). The Fourrier Transforms (and thus also the DCT) are part of the frequency analysis necessary to achieve that.