Pure audio assets w/o video are handled the same way as videos, see Single-File Videos [GL ARC]. The distinction between video and audio-only assets is made via mime types on shape level.
In an Enterprise MAM Solution audio assets also have proxy shapes for playback in frontends. This ensures that you have a uniform playback format in the frontend and allows consistent audio and video handling.
In other contexts (e.g. EditMate on VidiNet) a proxy shape is optional for audio assets. The UI may use any suitable shape for playing back the asset in the browser.
Audio assets should also have a thumbnail image that can be used in frontend for displaying audio and video assets in a consistent manner.