Same here (use serial number to protect Max/MSP app). Many people
also include some kind of way to "alter" the audio (insert silence,
random noise, etc..) in the demo version. Up to you whether you want
to create a separate demo version, or just a single version which
takes this "altered" audio out of the audio stream once authorized.
You can also time limit the software for each session (i.e. 15
minutes) until it's authorized. Just put a timer into your app.