Jack (the Jack Audio Connection Kit) is a low-latency audio server, written originally for the GNU/Linux operating system, and now with Mac OS X support. It can connect any number of different applications to a single hardware audio device; it also allows applications to send and receive audio to and from each other.
ZVocoder is a small application that will take two sound files and combine them into a new sound file through a vocoding algorithm. A vocoder is typically used to combine a speech sound with a frequency-rich musical sequence to generate a robotic-sounding singing voice. It has been used widely by many musicians.