Change the audio data of m4a or wav recorded by voice on iOS app to videos such as mp4.

I would like to change the audio data of m4a or wav recorded by voice on iOS app to videos such as mp4 and share it on Twitter and Instagram. If you know how to convert it, please let me know.
There is an app called Motiv Audio in the App Store that converts recorded audio (wav) into an mp4 file and saves it to a camera roll, so please let me know if you have any good ideas or advice.