BeSweet. This is *the* tool for transcoding audio. In my case, I have MP3s that are encoded from CDs at 44.1KHz, but to use it for DVD, you need to transcode it to 48KHz. There are lots of tools for this, but the most common one that is a standalone utility is besweet. You download besweet and then besweetgui.
Then, you click on the input as whatever MP3 file that you have and then make sure to click on the SSRC checkbox for Downconvert Sample Rate. This lights up the SSRC button in the Setup Screens section.
Click on the SSRC button and click on Set Sample Rate and fill in 48000. Finally click on MP3 to MP3 and it will transcode it into an MP3 that can be used in Video Explosion.