You can buy the DVSI codec wrapped up in a USB stick from NW Digital Radio. They call it the "ThumbDV". It looks like a serial port to your machine, and you can just poke it with the right commands and it'll turn raw codec data into raw audio data (and vice-versa).

I used it for decoding the AMBE+2 stuff on newer P25 systems. It wasn't great but it did mean I didn't run afoul of any patent issues if I wanted to sell the system to someone else. OP25 is great but I didn't want to take the risk.