This company seems to have successfully modified the internal workings of a Tungsten C and will do it for you as well. Cost is $69.99, and is guaranteed for 30 days…
We have modified a number of Tungsten C’s by adding an internal
condensor microphone and boy do they work great! This mod allows you
to make full use of the voice features built into the Palm OS 5x.
Enjoy all the benefits of the Tungsten T with the thumb board of a C.
I’m not sure if I don’t use the voice note feature of my Tungsten because there is no mic built-in or because I don’t generally feel the need for voice notes. I recall using them on my Newton MessagePad 2100, which not only recorded the notes but listed them on my “Today” screen like ToDos and memos…