so if you play back the media files in the source panel they have sound? just not the timeline? what if you drag the files back into the timeline, do they still have no sound?
there is a chance the project file became corrupt and lost the audio, in that case you may have to relink or redo the audio. if the audio is there, it should be visible with the "show audio waveform" and expanding the audio track as neil shows in his pictures above. if the audio is showing, just not audible then the audio track or master track may be muted or volume set to silent.