"RingtoneManager: unable to find a usable ringtone" persistent error

Some users of our app are reporting that ringtones fail to load as expected. I've included a typical logcat output from one of these users below. It seems that the request for the default ringtone is returning null ("Failed to open ringtone content://settings/system/ringtone"). I've asked these users to change ringtones, and sometimes this resolves the issue, but in other cases (HTC Hero in both the on开发者_JAVA技巧es I know of) changing the ringtone does not fix the problem. I have been unable to reproduce this problem on any of our test devices. Any thoughts as to what might cause this, how to debug, or how to resolve?

D/Ringtone(  743): create mAudio
D/MediaPlayer(  743): setDataSource(44, 0, 576460752303423487) pid = 743
E/Mp3HwPlayer(   56): ID:24, new Mp3HwPlayer
E/MediaPlayerService(   56): getPlayerType(): Has ID3 info, size = 1024
E/MediaPlayerService(   56): getPlayerType(): Input buffer is MPA format
E/Mp3HwPlayer(   56): ID:24, destory Mp3HwPlayer
D/MediaPlayerService(   56): Create Player playertype=6
D/MediaPlayerService(   56): new PVPlayer for MP3
D/PVPlayer(   56): setDataSource(25, 0, 180143)
D/MediaPlayerService(   56): MediaPlayerService::Client::setDataSource exit
D/MediaPlayerService(   56): returned from setDataSource() in MediaPlayerService::create
D/MediaPlayerService(   56): [23] prepareAsync
I/PlayerDriver(   56): OpenCore disable firewall payload
W/MediaPlayer(  743): info/warning (1, 26)
E/PlayerDriver(   56): Command PLAYER_PREPARE completed with an error or info PVMFErrResource
E/MediaPlayer(  743): error (1, -17)
E/RingtoneManager(  743): Failed to open ringtone content://settings/system/ringtone
E/RingtoneManager(  743): unable to find a usable ringtone
E/IncomingRinger(  743): Couldn't find a ringtone for URI: content://settings/system/ringtone

---

It looks like the ringtone is stored on a SD card, and you haven't permission to read SD data, so it returns null. I got similar problem with my mediaPlayer.