The tromboon (also called a babone) is a musical instrument made up of the reed and bocal of the bassoon attached to the body of a trombone in place of the trombone's mouthpiece, combining the "worst" aspects of each instrument, a reed and a slide. The name of the instrument is a portmanteau of "trombone" and "bassoon". The sound quality of the instrument is best described as comical.

http://en.wikipedia.org/wiki/Tromboon
be sure to click on the tromboon sound example

i know taht the link on the top works but the lik for a sound sample may not
http://tools.wikimedia.de/~gmaxwell/jorbis/JOrbisPlayer.php?path=Tromboon-sample.ogg&wiki=en

I must say, that was pretty awesome... I'm almost tempted to make one myself and see what I can get out of it. =P

I've tried putting a clarinet mouthpiece on a trumpet before with the use of some silly putty (horrible I know) it had quite the sound. =s