There are a few programs that can do this. There's SynthFont - the old version is a free download. If you have a soundfont, you could also use Spesoft Audio Converter (I personally recommend this one because the output is good quality)
You'll need a soundfont as an sf2 file. If you don't have one, you could try FluidR3. http://ftp.de.debian.org/debian/pool/main/f/fluid-soundfont/fluid-soundfont_3.1.orig.tar.gz