There are a few things you can try.
You can format your SD-card. Go to settings > storage > unmount SD card > erase SD card. Don't forget to move all the files you want to keep to your computer before you do this. Move back all the files and try again.
If the problem persists you can try to clear data for gallery/album, music player and media storage under settings > apps > the "all" tab. Then restart the phone and try again. When you have cleared data for media storage it take some time for the phone to read in all you files again.
And if the problem problem still persist you should try to update/repair the phone software using the program PC Companion:
http://www.sonymobile.com/update
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