Modern .mov files are Quicktime containers for mp4 (m4v) video files and some audio file.
It sounds like the current ones you have are using standard m4v streams, but the audio format has changed to something that the Samsung no longer recognizes.
Try
a) downloading the latest firmware from Samsung for the player
b) on the PC/MAC, download QuickTime player. Open the ones you cannot hear on the Samsung, and convert them by going to File --> Save As, or Export (depends on the version), and change the audio format to one supported by the Samsung.