In all reality, there are two ways to attempt to listen to Opensky, neither one very good.
1) Mod your scanner to use a Digital Speech Decoder and modify the DSD software so it can effectively decode the audio. However, you would have to be a highly skilled software programmer and have a lot of time to modify the DSD.
2) Steal one from a Squad Car or a Fire Truck. I would highly recommend NOT doing that AT ALL. That would be a one way ticket to prison with a nice charge of Grand Larceny.
In the end, your best chance is that Milwaukee will scrap Opensky like Las Vegas scrapped Desert Sky.
Because knowlege is power, a power that can save your life