To convert files, I am using Sound Forge 4.5, which does much more than this, but does also conversions, resample (changing the Khz rate), channel merge (stereo to mono), etc..
As for your second question... There are some modification tools to change the WAV sounds you hear on
the TR808 and 909. You can import any sound and create your own "mod" (you can even change the graphics). I think you'll need the full rebirth to do that, and the mod program. Go to the rebirth site (its listed in the music portal on this site) to find out more details.
I have the feeling you will LOVE reason... reason is the new program made by the creators of Rebirth, which will be out shortly (before 2001). It has many modules, including a polyphonic software sampler. A sampler let's you record any audio source, and mess with it, play with WAV files on your keyboard, add filters and stuff to it... Reason also has classic synthesizers in there, where you can create your sound from A to Z building with Oscillators, VCO, Enveloppes, etc.. I think you should get reason when it is out
A full review will be published here as soon as they give us a review copy.
Take care