well any speaker/earphone could be more directional. But their sensitivity are usually pretty bad too. Never tried with any earphone (walkman or else).

What range of frequency you need to detect?

One thing is sure, you'll have to use a high pass filter to avoid any type of interference caused by your robot motor/floor/gear noise.

I really have to do something like that one day. As many other i've buyed the Robosapiens, dismantle it... and let him gathering dust

And yes the mic signal is analog.

PS: you don't need to call me sir. I'm not as old as Darrel Taylor is