You can use the line~ object to supply a changing speed value to the right inlet of the sfplay~ object. The second outlet of the function object sends out a list message intended to describe the function to a line~ object.
The peakamp~ object can be used to take the amplitude of one signal and have it control the amplitude of another. Peakamp~ takes a low-frequency sample of the peak amplitude of the signal, which is then smoothed by the line~ object. This gives an intuitively much more accurate representation of the audio signal one is envelope-following than does smoothing the audio signal itself, which tends to smooth out — and thus lose — all the more interesting characteristics of the signal. Here’s an example that lets you compare.
A named record~ object records audio data into the buffer~ object that shares the same name. When the record~ object receives the message 1 (or any nonzero number) it begins recording the audio signal it receives, by placing that data in the associated buffer~, starting at the beginning of the buffer~. When record~ receives a 0 message, it stops recording.
The way we commonly avoid clicks when changing the amplitude of a sound is to interpolate smoothly sample-by-sample from one gain factor to another, using an object such as line~. Does that same technique work well for making a smooth change from one delay time to another? As it turns out, that's not the best way to get a seamless unnoticeable change from one delay time to another, because changing the delay time gradually will actually cause a pitch shift in the sound.
One possible solution to the problem of clicks occurring when delay time is changed is to fade the amplitude of the delayed sound to 0 just before changing the delay time, then fade back up immediately after the change. This does avoid clicks, but causes an audible momentary break or dip in the sound. This shows one way you could implement such momentary "ducking" of the amplitude.
This patch shows several techniques relevant to granular synthesis, playing a stream of short excerpts of recorded sound. (The patch uses one abstraction, called pan~, that's provided in the example titled "Constant-intensity panning subpatch". You'll need to download that abstraction and save it with the name pan~ somewhere in Max's file search path.)
Will need to download the pan~ abstraction found here.
For this example to work properly, you must first download "Abstraction for mixing or crossfading two audio signals" and save it with the name xfade~.maxpat. Then you must ensure that your xfade~.maxpat file is in the Max search path, or you must save this example patch in the same folder as the xfade~.maxpat file and reopen it, so that this patch can find the xfade~ object.