If you have an audio clip that is very quiet in some parts, and very loud in others, it’s hard to work with it. You can break the clip down into segments and increase the volume for the quieter parts, or decrease it for the louder ones. This is tedious and makes for hard work which is why there are apps that can normalize audio in a file. A good, free app that can normalize audio in a file is Audacity.
Download and install it, and if needed, install any libraries you might need to import your audio file into the app. You should also read up on the types of normalization methods for audio. You can also just go with loudness normalization which normalizes the audio file by how loud it is.
Open Audacity and import the audio file that you want to normalize audio for. Select the entire audio clip if you want to normalize all of it, or select a section of it to normalize. Go to Effects>Normalize.
In the window that opens, select the loudness level in decibels that you want to normalize it to. You can change these levels if you know what you’re doing but the level suggested by Audacity will be accurate. Click OK to apply the normalization or click Preview to hear a snippet of the file and see how much of a difference the normalization has made.
Working with audio is always tricky and normalization alone may not do the trick if there is a huge difference in loudness in the clip. If after normalizing audio, the clip becomes too quiet, you can try using the Amplify effect to make it louder. It might also be a good idea to remove background noise from the clip before you normalize it. Normalization has to do with loudness and while the app will try and keep the quality of the audio from deteriorating, the normalization might still compromise it.
You can try using paid apps to see if they yield better results but Audacity is one of the best apps in the audio file processing market. Just because it’s free doesn’t mean it’s less capable and professionals do use it over other premium solutions.
If all else fails you might have to record the audio again. If it comes down to that, determine why the volume within the clip varies so much and eliminate every possible cause before you record again.