Help Samsung Galaxy SII - Music Files won't appear in Music Player


I'm having a slight problem.

Usually to update my music library, I use the USB lead to import music files from the computer to my phone and I put them in the 'Sounds' folder. Normally upto now, they've all been imported correctly and they play fine in the music player. Today, I was adding more songs into the 'Sounds' folder and I deleted some that I didn't want; when I disconnected my phone from the computer, the files that I had deleted were still in the music player but not in my 'Sounds' folder and also the new files that I had imported did not appear in the music player like they usually do. I have tried everything, from switching my phone off and back on to deleting the files from the 'Sounds' folder and adding fresh new ones but still nothing has happened.

If someone can please help me, that would be great.