Dear Manuel,
The developers went through this again and I got a possible solution back, this is his response:
The very easy way to achieve this is not to use a checkbox but "checkboxes", with a single defined value instead of auto_fill, more or less like so:
[wpv-control field="salary" url_param="audio" type="checkboxes" values="some_value" display_values="Some value"]
The resulting query string is exactly what he is asking for.
Please let me know if you are satisfied with the answer and if I can help you with any other questions you might have.
Regards,
Caridad