Is it possible to make a patch in Pd that recognises the start of an incoming audio and translate that into a midinote or a bang?

I'm thinking specifically of a contact mic plugged into the audio-in of a laptop. When the contact mic is knocked, it triggers the patch via a bang.

Any suggestions? adc~ into mtof seems logical perhaps?

you should look at bonk~ or fiddle~

basically adc~ => bonk~ => mtof => synth

You could also look at the [env~] and [envrms~] objects. You could use these (together with [line~] to smooth them) to detect peaks.