I read about how to play a sound using the MediaPlayer API, however, I have no idea what file extension I should use. It is for a custom notification sound, and I'm sure MP3 could be the way to go, however I'm not sure if there's a convention to a type that is better for some reason.
I know this question might be more of an opinion base, but in any case, let me know what you guys usually use.
I'm not sure if there's a convention to a type that is better for some reason
No, there is not. MP3 should be fine. You can find the supported media formats in the documentation.