Like bouteloua said, you can actually upload just sound bites as an observation. Annoyingly though, and I think what you’re getting at, is that when creating a new observation from within the iNat app, you can only take photos, you can’t record audio natively from the app.
The best solution I have for this is using Merlin to record a song. I hit save, and then, after saving, I click the share button. On android, that button allows me to “share” to another app, and iNat is an option. This will automatically create a new observation with just that audio file