Thank you for saving me the trouble of investigating this as an option.

No reason to tolerate proprietary licenses when there are so many viable FLOSS solutions out there.

The github page has more context than the F-Droid listing.

API Key

This application uses the AudD® service as a Music Recognition API. You need a special API token provided by AudD® to use the application. If you don’t have one, you can sign up for a free API token. You can add the key on the onboarding or preferences screen, or just set it in local.properties.

There is also the option to use the app without a token, but please note that this will restrict the number of daily recognitions that can be performed.

The AudD web site says:

Music recognition API for both content analysis and in-app music recognition costs from $2 to $5 per 1000 requests. First 300 requests for free.