Here's some instructions for ya:
1. Download the LAME MP3 ACM codec or Radium MP3 ACM codec(Google it)
2. Install it
3. Find a wav file you want to convert and open it in "Sound Recorder" (the one that comes with windows).
4. Click Save As...
5. Click the "Change..." button next to "Format:" at the bottom of the save window.
6. Change the "Format" dropdown box to MPEG Layer-3
7. Select the bitrate, frequency, channels below that under attributes.
8. Save the file as a .WAV.
9. Done.
As far as I know FL will handle any codec as long as it is in the WAV container and the computer has the appropriate windows decoder installed. But Mpeg-3 works best for size/quality. But make sure that you pick CBR (constant bitrate) and NOT VBR (variable bitrate). This option made FL crash on two of my computers so I assume it doesn't work
I might write a complete tutorial soon as nobody seems to have done this stuff.