Hi J,
I used the electret microphone from an old cell phone. It is very small and works very well.
If the phone has a vibrating ringer, check-out its motor. It is probably the smallest motor you've ever seen. I kept looking at it, wondering what it was until I removed it and its cover. It swings a weight around and around.
Maybe you could also use the phone's little speaker for a beeper or something.
I know, you could use the mic and speaker for a little keys-finder project. The mic hears your whisle or clap and activates its speaker as a beeper.