I want to burn 15-20 CDs onto BD and play them in my external Pioneer BDP-320 Player. I don't need video--I just want to save space and have crystal-clear audio to listen to. I have an internal Blu-Ray burner.

According to your owners mmanual you can play audio files. So all you need is software to burn it to your disc. Media Player (if your using windows) has the ability to do it as well as other commercial software. They mentioned "home media group" in the manual.

Ref:

http://www.pioneerelectronics.com/ephox/StaticFiles/Manuals/Home/BDP-320_OperatingInstructions0223.pdf