MeowMultiSound is a free sound converter that supports mp3, wav and ogg sound files. It allows to convert files in both direction. Features drag and drop support and an option to generate Id3v1 tags common to several files.
Midimeow is an easy-to-use tool to convert:
Midi to Mp3
Midi to Wav
Midi to Ogg vorbis
It allows to change the rate of the output (3 rates available)
The number of channels (mono or stereo)
And if you want the volume