This has pretty much been happening from day 1, but possibly only since FroYo. I have some ring tones and notification tones on my SD card. My Google Voice text uses one of them. At some point that tone stops working and then all I get is vibrate. If I check the tone it appears on the list but will never play. In fact all of the ones on the SD will preview as the same generic built in tone. However messages don't play anything. After that has happened all media found on the SD will fail to play, including movies or audio files. Even though the ES file browser can see them. Only a reboot will restore function. Though I have not tried connecting the USB cable to see if a dismount/mount cycle also restores function. Any if this sound familiar? By the way I HAVE copied wiped and restored the SD card. 
