I believe it might be possible to use an open source codec, depending on software patents in your country, but I went for the digium codec. It's licensed per concurrent channel. There are instructions on the digium site, you download a tool which runs some simulations and finds the best build of the codec for your cpu. You download the appropriate codec and stick in the folder as instructed. Then there's a register procedure that creates a license key based on the purchased I'd and the MAC address of the server. The whole process, following the digium instructions, was painless.
But it does all need to be done from the server cli, so ssh or terminal access.