I'm having trouble with the Music app on my EVO, I downloaded several albums from my cloud storage, no problem, opened the music app and it found all. I then later downloaded a new album and it does not show up. The only way I can play those songs is by using my file manager. All my music is stored on my SD card.
Try the following,
Settings > Storage > Unmount Storage Card
When it's finished, it should say something like Mount Storage Card, press it again. The system will then scan all files on the SD Card and recognize images, videos and songs.
WARNING: If you have any of the apps or camera set for Auto Upload, it will upload any new images found to those app sites. Heck, it may just re-upload all images, not positive. In any case, if you don't want that to happen, deactivate the Auto Upload feature for the apps and camera for a day or so.
Is this a brand new SD Card or one from a previous phone? Some people have noticed this issue and more when they brought it from another phone. Usually, moving all your data, reformatting it and then putting your data back resolves. But not sure if you want to do all that.