1. A portable mixing recorder comprising: an input and conversion device that receives an analog audio signal and converts the analog audio signal to a first digital audio signal; a decoding device that reads out a compressed audio signal from an original source file stored in a predetermined memory and having the compressed audio signal recorded therein, and then extends the compressed audio signal into a second digital audio signal; a mixing device that mixes the first digital audio signal and the second digital audio signal into a third digital audio signal; a first encoding device that compresses the third digital audio signal into a compressed audio signal; and a storage control device that causes the compressed audio signal obtained by said first encoding device to be stored in the predetermined memory, as a new source file.