With a supply of 3 - 9 volts it will drive and earbud or headphone. It could be a good starting point. I think a 2N7002 would work without any circuit changes?
I'm using the K669 because that is what I have on hand in leaded MOSFETs. My 7002's are surface mount, I may try one with them. A match box amp would be good to have around.
* you can connect the earbud / headphone directly to the source. I don't like putting DC thru mine and I have a bin full of the little transformers so I use them. You can wind your own. A 1:1 or 1:2 will work.