Hello and welcome to the forums. Even though the video player app will indicate that the volume is at whatever you set it at, the tablet is automatically audio muted when an HDMI cable is plugged in. Plug in the cable with the tablet on, go to Settings via the icon and unmute the sound. The volume setting will go to 100% and the HDMI sound output to the TV will work while the tablet's speakers will actually still be muted, a good thing since when the TV audio is used, there's no reason to waste the tablet's battery power on audio.
Note**** So, the proper sequence is, turn on tablet and let it boot, plug in the HDMI cable, go to Settings via that icon and unmute the sound, run video player app, run videos. If the sound doesn't work at that point you have a defective Sero 7, a bad cable, or the monitor doesn't work with the output of video decoder of your video player app is using