MP4 is just a file format. It describes how data is packed into a file, but doesn't tell you how it's been compressed. Your file uses video codec that is supported by your players and an audio codec which is not. Your players can't decode sound. That's also the reason why you don't hear sound after splitting the file - the program you used didn't support that codec, so it has just ommited sound entirely.

As @BrunoPereira suggested in the comments, you can use MediaInfo to get some details about the codec used.

MP4 is just a media container. It can contain different media streams, for example audio, video and subtitles and allows to keep them conveniently in a single file. 

Your file contains a video stream that is supported by your players and an audio stream which is not. Your players can't decode that audio stream. That's also the reason why you don't hear sound after splitting the file - the program you used didn't support that codec, so it has just omitted sound entirely.

As @BrunoPereira suggested in the comments, you can use MediaInfo to get some details about the codec used.

After you've found out which codec is used, you can install one that is compatible with your OS and media player.