This shows how to record into a sound file, and how to play back the contents of the file. The example also demonstrates how you can use a quick fade-in and fade-out to avoid clicks when doing realtime capture during a performance.
For linear interpolation of a MSP signal, the line~ object sends out a signal that progress to some new value over a certain amount of time interpolating sample-by-sample along the way. The input to line~ is a pair of numbers representing a destination value (where it should eventually arrive) and a transition time (how long it should take to get there). It can receive multiple pairs of numbers in a single message, and it will use the pairs in order, starting each new pair when the previous transition has finished.