I think it is now included in the Sonar update and just will work automatically after you update. But Home Studio has not been updated so it is not available yet.
I use Gold Wave to batch process to MP3. Audacity and Wavosaur also have free mp3 conversion.
I want all my songs stored as high quality WAVE files anyhow, so it's just as easy to batch convert as to re export as an MP3 directly.