Start with the unedited file. Make a working copy before changing volume, removing noise, or cutting a clip. Record the timestamp and what you remember hearing on location separately from what you hear during playback.
Ask another listener to describe the sound without first telling them your interpretation. If you say “it says get out,” you have changed the listening task. Compare descriptions afterward. Agreement may be interesting, but agreement alone does not identify a source.
Check ordinary sources that were present: conversation, clothing against the microphone, footsteps, a radio, passing traffic, and automatic gain changes. Document what you can check and what you cannot. Avoid treating an unchecked possibility as an excluded explanation.
Enhancement changes a recording. Keep a list of processing steps and publish the original alongside an enhanced excerpt when possible. If a phrase only appears after aggressive processing, describe that dependence clearly.
A useful caption can be simple: “Unidentified sound at 04:12. Recorded during a solo walk. Source not visible. Clip amplified; original retained.” It lets viewers encounter the evidence before your conclusion.