"Hiss", or White Noise heard on video playback is quite normal. It is present on all recordings. It indicates that the level of the sound, when recording the video was too low.
There are two ways to reduce it.
(1) Ensure the audio input is at a reasonable level, without overload. This is very difficult to achieve on a simple video camera on a phone with a built-in microphone. Video cameras have facilities for an external microphone and or a built-in directional one. This explains why a voice memo exhibits less hiss because the microphone is so much closer to the speaker.
(2) Reduce the playback audio volume. If the original video was recorded at a good volume, playback volume does not have to be set at too high a setting, thereby increasing the "signal to noise ratio".
In a nutshell, don't forget the video camera on a phone, like the still camera, is a very simple and basic item so don't expect it to compare with a dedicated video camera. Try to keep the record volume up and the playback volume down.