You can add an audio file that you have already placed into your Drive or you can attach an audio file from your computer.
When the student opens the audio file in Google Classroom they are going to get a 'Whoops! There was a problem displaying this image.' All the student needs to do is click on 'open' at the top of the screen and the audio player will display.
Audio files are great for your students who might need oral administration. You can create audio files using Vocaroo. Check out my tutorial on this tool.
If you need extra assistance with Google Classroom, please check out my resource that shows both the teacher and student view.