Currently, in the Canvas Student app, there is not an option to upload an audio file to a Media Submission assignment type. Students can do this on a mobile browser or on a desktop browser, but are only provided with Record Audio, Record Video, and Upload Video through the Canvas Student app.
If students use the File Upload assignment type, the audio cannot be opened by SpeedGrader or the Canvas Teacher app. Students can use the URL Submission, but the audio doesn't play natively in SpeedGradoer or in Google Drive. The teacher would have to download each file individually.
While it works for students to use Safari to submit the audio file they have saved on their device, it would streamline the process to have Upload Audio available in the app.