Having just gotten my first PocketPC I wanted to make the most of my 256MB SD card for music purposes.  I guess it was slightly disappointing to me that I only got 58 mp3s on one card (SD cards just came out in 1GB versions BUT they are still a bit pricey).  Fortunately there are many easy, free solutions for converting mp3s to WMAs which generally seem to offer better compression for similar sound quality.

The free Windows Media Encoder offers a Windows script that will batch convert media from mp3 to wma.  While this isn't ideal ('cause both are lossy compression methods and going from lossy to lossy will tend to be worse than re-ripping the original source) it is a decent lazy man's way to more music in limited space.

A simple command line like the following will convert all the mp3s in a folder:

C:\Music>cscript "c:\Program Files\Windows Media Components\Encoder\WMCmd.vbs" -input .\ -output .\ -profile a96

Here the options simply specify the script that comes with the Media Encoder (WMCmd.vbs), the input and output folder (the current folder) and the a96 argument of the -profile option signifies the bit rate you want the output to have.  I've found 96kbps to be sufficient for similar quality while saving a lot of space.  I got my 245MB of mp3 down to 175MB giving me maybe another 15 or 20 songs.  The only drawback is how slow the conversion is, darn VBScript.