Hi can anyone please help me...

When in ths sequence window, I double click on a sound bite and highlight a peice of the sound bite(as I have always done with no problem), and the I delete the selected part, that part dissaperes and the sound bite moves way out of time with the rest of the song... I know it must be some setting because it only happens in the one project I am working on... Any ideas please help if you can thank you JASON