I connected the Droid Charge via the microUSB to USB cable to my homebuilt Win 7 powered computer. Using Windows Explorer I created a Music subfolder on the SD Rom's root folder. Copy/Pasted a bunch of .mp3 songs there. I have no problem selecting any of these songs as a ringtone. Plus, the Music Player automatically finds and plays these .mp3's. I also use A/V Replay program to record live radio shows, convert to mp3 and they too work fine. They are in a Radio subfolder I created.
Movies ripped to .mp4 are placed in another subfolder I created with Windows Explorer named Movies. Video player automatically finds and plays them just fine. All three folders mentioned were placed in the root folder of the SD Rom.