If you are storing audio files on the hard drive they often need to be refreshed. When you moved the case you may have tugged a plug out a little causing intermittent sound from that. Often it is either the files themselves or the player used that somehow loses information.
To find out if this is what happened you could try a different player to see if the problem remains. Hopefully you didn't tug on the cables causing the sound card to lift up in the slot. Or you may have to reinstall or update sound drivers. There are a few different things you could look at here.