If an observation has a sound file and no images included, the icon on a project page is the “group” icon, which makes it look like the observation has no visual/audio data (a Casual ob):
Project link - https://www.inaturalist.org/projects/bladder-grasshopper This is a collection project. I have not been able to check the icons on Traditional or Umbrella projects because I couldn’t think of any offhand which would have a sound file on them.
Whereas, it should have the sound icon so that viewers will know there is data, and it fits in with the rest of the icon usage on iNat, like on the Explore page:
These are representations of the same observation. And I have checked it - there is a sound clip, and it does play :-)