WARNING: the app doesn't save the voice notes. If you delete the files from WhatsApp or from a file manager, they can't be recovered by the app. Opus Player was born as a simple audio player for the opus format, but now it is something more. As most audio players, you can start it in your file manager, but differently from the others, you can choose the audio output, selecting the main speaker or the earpiece (the speaker for the voice call). It is useful if you don't have a headphone with you.