Volume level too low
You may be surprised, but the most popular fix for that is to remove any physical objects that are blocking the microphone or speaker.
So make sure you know the location of these important parts of your device. Try not to block it when holding your device during recordings. If you use a protective case, make sure it's not the cause of a bad experience.
Some devices have special sound processing algorithms built-in, just go to Settings -> Sound and check it out.
As a last resort, you can use a Mic adjustment feature. It can multiply the volume of a recording but with some additional noise as well. You can find it in the app's Settings.