MIDI does not synthesize the human voice. On the minus side, everything over a few seconds of complex music can produce HUGE files. Voice WAV files are often sampled at 11 KHz to save space. The WAV format allows some compression, and AU files provide better file compresson. To create WAV files you need a program called a mixer, and sound sources.
Most sound cards come with a simple mixer.
The card has a line input connection for an amplifier, and a microphone input connection. The mixer program will also mix analog sound with sound from an onboard MIDI sequencer. Once you have WAV files, you can use a wave editor to actually cut samples from one file to another, and to modify the sound of the file. The display of the editor looks like the trace of an ocsilloscope, except that you can freeze it and mark the part of the trace you want to work with.