Currently there isn't an option to upload audio (.mp3) files. However, you can upload video files to skills assessments. As a workaround, you can use a converter that allows you to convert an audio file to a "true" video file that uses an image.
Attaching the image tricks the system into thinking the audio file is a video. We recommend using a free site like http://ez-converter.com/, but any site that allows you to attach an image to your file before converting it will work.
Once you convert the audio file, you can go back to editing your assessment and change the question type to video. Then you will have the option to upload the converted video file.