Some "cam" sites stream their videos using non-standard audio and visual combinations. Consequentially, while Replay Media Catcher will save the entire stream, it appears as though there is no audio present when played back. The simplest way to reveal the audio is to do a conversion to another video format such as MP4 Default (MP4).
Many of the MyFreeCams feeds use an incompatible audio + visual stream format. While the audio is saved in the file, most players cannot handle the combination. A conversion to MP4 will take care of it.