Conclusion

Use extension .ogg or .oga instead of .opus.

Description

Android inplements Opus codec from Android 5, but can’t play .opus file.

Opus audio is included to Ogg by default and .opus extension name is recommended, but Android didn’t recognize .opus as audio file.

oga is generic audio file extension on Ogg. ogg is old generic extension on Ogg. They are recognized by Android.