Any given MSP patch cord represents a single channel of audio. If you want to generate or process multiple sounds or channels, you need to treat each sound or channel separately. For example, each sfplay~ object can have multiple loaded sound cues so that it's ready to play any one of several files, but it can only play one sound file at any given instant. And if it's a stereo file you need to treat each channel separately for mixing, processing, etc. This patch demonstrates that.
If you want to change the coefficients of biquad~ in real time while a sound is playing, it's usually better to use MSP signals rather than individual Max messages, to avoid causing clicks. In that case, you should replace filtergraph~ with filtercoeff~ and send the frequency, gain, and Q parameters into filtercoeff~ as smooth signals (as shown in the left portion of the example).
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.
The right inlet of sfplay~ accepts a rate factor (increment factor) to control the speed of sound file playback. The factor can be supplied as a floating point number or as a MSP signal.