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.
No, there is not. MP3 should be fine. You can find the supported media formats in the documentation.